I've played a fair bit as a summoner now, using this mod. Got to say that it's an amazingly well put together mod. No bugs encountered and everything felt as it fit right into the world of elemental. Felt as if it could had been part of the game from release. Unfortunately it sort of got boring pretty quick due to the lack of balance.
As mentioned by others it's a lot easier to powerlevel a summoner then it is to tech up to equally strong units. This gives a snowball effect as each extra level your summoner gains gives you an even stronger creature to help you level up even faster.
I've got a few suggestions, tough I'm not sure if they are any good, since it's basically pokemon.
1. Decrease the number of summonable units to just one per school. Just for now. You've created some very interesting units but six new units are plenty to playtest and give feedback on. Also, I wouldn't mind if you'd started by tinkering with the vanilla summonable units first and then added new units once you feel the old ones are fun and balanced. I found no reason to use the vanilla summons at all with your mod installed.
2. I personally would have prefered if the units you summon start out at just lvl 1 and quite weak. And then become stronger than conventional units as you level them up. You've already added some abilities that are not featured in any vanilla unit or spell. If they would increase in potency as you level the creature up, instead of starting out fairly strong, that would work I think. This way you can summon units all day but you'd end up with a really weak army that would only really be good for emergencies. Or focus on training a few to become endgame powerhouses.
3. The horde boosts to summons is a great idea. But perhaps it's better to have those units to be upgradable by paying 50 mana for each extra figure in the unit. This is because there's no reason to use lesser variants of the horde-type summons once you gain the stronger versions. And if you already summoned a bunch of the lesser versions they are essentially wastes of xp in the later game. 50 mana is a great deal for just a slightly stronger unit. But if it is a means of boosting your precious summon, after it gained a couple of levels, it would be well worth the cost.
4. Consider making a few of your units something like henchmen. They add a slight boost to the entire army and can use weapons (or shields for the earth version) and misc items. If they get a bonus to their brand of elemental damage (say +20%, earth gets a defence boost instead, life gets a chance to heal afflictions on itself and death ignores 20% armor) it would push the player to give them items corresponding to their theme. This way you can make the unit itself weaker since the player can add items to boost them.
5. Consider what each type of magic is lacking. For example, It's tempting to make the fire summons weak in hp but strong in damage. But with so many spells that do damage fire mages don't really need something more that adds damage. The need something that ties up the enemy long enough for them to cast their spells, like a big unit with massive hp that simply blocks the path of the enemy. In contrast water mages lack a good artillery spell and so could use a strong archer-type unit. Life needs something that does AoE damage or boosts army damage, death mages need a slow tank (grave elementals are already a good fit), earth could use a fast but fragile assassin and air a weak medic. This way the summoner would do similar damage to a standard mage, but in a slower, more flexible way.