Ok, let's imagine three 2v2 teams. Which of the teams is probably the best?
- Oak/Sedna
- Oak/UB
- UB/Tower Rook
Despite the fact that UB is generally considered one of the stronger demigods, it's Oak/Sedna that is probably the best 2v2 team in that situation. Why? Because of synergy.
Synergy Basics:
Synergy is basically the ability of your team to work well together. If your team is full of powerful demigods that don't work well together, it could lose to a team of underlevelled, underquipped demigods that do. I will define two types of synergy; level synergy and skill synergy. Level synergy is essentially the synergy that comes from having your Demigods all be at their peak at certain levels; skill synergy is the synergy you get from your skills complimenting each other.
Level synergy:
Level Synergy is just my fancy way of saying your Demigods are all effective at the same level. Why is that useful? Because it means that, since both teams generally stay even on levels, your team is at its strongest all at the same time while the enemy may have weaker links you can pick off. Take an Ooze UB and a Mines regulus; they both get their best skills at level 10 (maximized ooze and shrapnel mines). They also have pretty good skill synergy in other areas as well, but I'll mention that later. Another good combo is TB and UB; TB is effective at level 10, and probably can get a couple levels higher than anybody else, so they can easily start pushing at level 10 when they have a big advantage, and then use the push to gain more advantage because the enemy team may not be very strong at that level.
An example of bad synergy is an AA/Snipe Regulus and a Tower Rook. AA regulus takes a ton of gear and levels to hit his peak, while a Tower Rook hits his peak early because his towers stop being a deterrant to people with even moderate HP gear that can be obtained simply by the basic gold flow at around level 7. At any given level, one of them is weak due to how well (or not) their skills scale, so it all ends up badly. They also have horrible skill synergy, but once again that's for a later section.
So if you are making a team, try to find one that makes sure all of your Demigods are good at the same level, or roughly the same level, as each other (EX: if you have a TB build that makes him peak at level 15, but the rest of your team peaks at 10, it is probably OK because they will still be good at 12 and you can level up faster than them so you will also be OK.) As long as you do that your team will be able to make "miraculous" comebacks (in the enemies eyes) because they think that they are stronger than you just because of gear, which isn't always true. (of course, you could also just slaughter the enemies the whole time and just slaughter them more at your peak levels.
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Another thing about level synergy is that it is incredibly map dependant. On smaller maps, and more open maps (Prison and Crucible... very bad maps) you want a team that hits its peak as early as possible, because the enemy simply can't farm at all on such maps as long as you can out-fight them. Level 5 is generally a good peak for those maps; even if you can't beat the enemy at level 1, getting to level 5 is still pretty easy and generally stuns and other nice options unlock there. On medium maps, you want a peak at about 10; if you peak at level 5 on cataract, the enemy will just lanechange and farm up, and if they are grouped together you can't take both creep lanes. If you are on a huge map, 15 is the way to go; a lot of hugely powerful upgrades to abilities become available at level 15, and with how big the map is, it's not hard to creep farm your way there, though if you can get a team with good skill synergy to push, you can probably shoot for level 10.
Skill Synergy:
Skill Synergy is my fancy title for skills that work well together. Going back to my earlier example of a UB and Regulus, let's look at their strengths. UB and Regulus both have high speed. Both of them have a slowing attack or two. UB has a stun. Regulus has good AA damage and range. UB has good autoattack damage and can debuff the enemies AA. Regulus can make it extremely bad for the enemy to use skills.
Can you see the synergy there? They can both slow the enemy, and since it stacks, an enemy targeted cannot get away. They are both fast, so they can keep up with each other. UB has a great way of keeping the enemy from doing much autoattack damage, and Regulus can combine that with MotB to slow the enemy further or make them consider not using skills (three mines, maim, MotB, and diseased claw slows the enemy down to almost immobile). The main thing there, though, is that they both have great ways of keeping the enemy slowed/stunned. For example, during UB's grasp, regulus can lay mines. Normally, a quick enemy can avoid at least one of the mines because they take a bit to detonate, but while being grasped the enemy is stuck and unable to escape. Now with the enemy slowed, UB can keep him in Ooze so he can't escape and can't deal damage.
That's what Skill Synergy is all about. With a good combo, you can keep the enemy from being able to do anything at all with stuns and slows from multiple Demigods. For example, Stuns normally don't do much damage, so they really only are good as long interrupts (though some have natural combos, like shatter frost nova or bash the enemy for three seconds with mass charm). But with a teammate with a stun, it is stun + free DPS for the teammate, stun + free DPS for you, and your cooldown is probably getting close to another stun. You can synergize any skill, really. The most common synergies, however, are stuns and slows, and for good reason; chaining stuns and slows guarantees you a kill, while doing something like synergizing heals and shields can make you invincible, you still have to rely on outspeeding your opponent to actually get a kill, while your opponents probably can't do much against a chainstun, because it will be 3 against 2 from the first stun onward (and a lot of stuns are group stuns!)
An example of a non stun/slow synergy is the infamous "5 regulus sniper team." One shoots an enemy, all five snipe (tracking bug.) The enemy dies, or has to burn a teleport scroll to make it back to the crystal before the second volley (or burn potions).Most all teams with multiple of the same demigod have this kind of skill synergy, because all of them can do the same thing. Five fireballs, five foul grasps, five mass stuns and 10 bites, etc. They all work well because they all stack and keep the enemy preoccupied. A favorite of mine (though annoying if it is used against me) is the five last stand oaks team. Basically, they max out soul power and Last Stand, park a bunch of spirits at their base far from harm, run up to the citadel and start bashing on it. When they die, they get amazingly buffed attack to bash on the citadel. Five oaks can kill it in two-3 volleys at base health (with Soul Power and a ton of souls, oak can hit crazy DPS, and having it given huge buffs to attack speed and attack power just makes it even more absurd.)
Another, less "5 of the same guy" example is Sedna and Oak. Those two together are a very tough team. Whenever any of them is at low health, Sedna heals, and if things get tough Oak can burn a shield to help one of them run or stay in the fight, and Sedna can heal through the shield so when they come out, they are no longer weak. It's a great combo, and the fact they both have fast interrupts that deal good spike damage and are fairly fast just helps even more.
An example of bad synergy is the sniper/AA regulus and Tower Rook combo. Let's see why! Rook is slow, and practically immobile because he can't leave his tower farm. Regulus, early game, has to rely on snipe kills to kill demigods. He can only get a snipe kill if his teammate fights, which a tower Rook won't because he has to stay in his tower, so the only way for regulus to snipe is to get an enemy dumb enough to stand in a tower farm for a while. Then you have to realize that a tower rook probably has no stuns to keep them there (they generally focus on shoulder upgrades and towers, not Boulder Roll), and neither does Regulus, so you can't even make full use of your tower farm (high sustained DPS by being stunned and bashed while inside of it.)
Also, you want your team to have at least a secondary focus on taking down towers. While any group of three demigods can probably just bash on a tower until it dies without a fatality occuring, if you can bash towers safely and don't have to sacrafice demigod fighting synergy, the enemies have to fight your push, and guess what? You can kill them. For example, a team of Regulus, Torchbearer, and Spit UB is incredibly good against towers. They aren't the best synergizing team for killing demigods, but they are still really good at it, so you can essentially take down towers at your leisure and have a pretty good ability to chainstun and slow demigods that try to stop you.
So you always want good skill synergy. Two stuns? Good! Three stuns? Great! Three stuns, three slowing moves, and good AoE from all teammates? Amazing! Three stuns, three slowing moves, Good AoE, and the ability to take down towers safely? You win! Just make sure that your skills work out well together, and you should be fine.
Tying it all together:
Ok, so you want Level Synergy and Skill Synergy (though personally I feel Skill Synergy is more important). But you still need some other tips. First of all, none of this matters if you can't function as a team. It doesn't matter if you have a perfect chainstun build that can gank the enemies at level 4 and doesn't lose its power for the rest of the game, if you are all fighting in 1v1 battles in each lane, synergy won't do a damn thing for you. Work together!
Here's an example 3v3 team that really ties together all the elements that make synegy great. Torchbearer (fire and ice; fire for Fireball, Ice for Ice nova, Ice rain, and Shatter.), Erebus (assassin; bite, batswarm, and mass charm, mist optional), and UB (Pure Ooze). Let's see; they all hit their peak around level 11 (my TB build usually hits its peak around 12, but he levels up quickly anyway). TB gets his level 10 upgrades to skills and Ice Nova by level 12. Erebus gets his most advanced Bite, an improved Batswarm, and the last stun time upgrade for Mass Swarm at level 10-11. UB gets max Ooze at level 10, and maxes out all of the skills needed for the build (besides acclimation) at level 11. So they all are really, really strong at the same time.
Next, their arsenal. UB slows attack speed and enemy movement speed, and has a stun. Erebus has a stun and slows enemy movement speed and Armor. TB has a stun, a slow, an interrupt, and a spike move. The combo? Ooze on, Foul grasp, Bite + Fireball, Ice Rain, Ice Nova, Shatter, Mass Charm, Foul Grasp again. The damage of this combo is enough to kill pretty much any demigod not specced entirely for health. It's absurd. Not only that, but only UB's stun can be interrupted, the enemy will be too slow to escape if that happens anyway, and Mass Charm and Ice Nova are mass stuns, so any enemy minions will be stunned for a huge amount of time, and all enemy demigods will be chainstunned for 6 seconds by the two moves. The enemy demigods will have no escape, they will be softened up by a 40% attack speed debuff by Ooze, they will be hit by AoE attacks and AoE stuns, and so in the end, three enemy demigods will quickly become two enemy demigods at three quarters health with very low speed.
Hopefully this guide helps you out when figuring out your team composition. I don't expect it to help much in randoms (people generally pick whatever character they like, and teamwork is nonexistant), but when playing games with people you know, try synergizing! Trust me, it will improve your game way more than any build order or item guide will.
Some other synergies by Transitive ( 
for him): feel free to submit your own!
1. Unclean Beast with Foul Grasp and Rook with Hammer Slam. pin them in place and then drop the hammer. massive burst damage combo. (how did I forget this?)
2. Queen of Thorns Bramble Shield and Torchbearer's Fire Spells. the Fire AoEs are both very effective and high damage but are centered on TB himself so they put you in alot more personal risk to use against enemy DG's than you might be comfortable with. Bramble Shield solves this problem alot.
3. Sedna and Frost Torchbearer. can chain interrupts, stuns, and silences to keep an enemy team mostly ineffective for long periods of time. require expert timing and coordination to do it correctly though. (Really, really tough and requires Sedna to forget about healing. High risk, high reward.)
