getting steam rolled by enemy ai

Not enough info to tell you what you did wrong, but if you're still fighting it, mobility has always been the AI's weakness. Use movement to avoid his huge army and take all weakly defended cities. Make fodder to weaken his remaining army and then finish him off.

You will need to increase your Tech points for research each turn to catch up, but you need Parties/Squads and/or companies researched and Weapons up to Maces and Leather armor minimum.  Also you need Barracks and Command posts on your big cities producing your military engine.  Otherwise keep your heroes away from his big guns, AI do not defend well so go on the offensive and attack its towns I like to keep them if same faction and I can defend them otherwise delete each building and then RAZE(X) the settlement square itself to hurt the AI's economy.

Archers are always good.  A couple squads (or larger units) of longbows (no armor needed) can destroy most AI units I've run across especially if you have the crystal to pump them up with rings/amulets/etc... that boost attack 90+ ranged attack is so much fun.  Throw in a few Ice Lords or some other high attack melee unit for when things get close to you.  Always target anything with a ranged ability first and try to lure his melee units on to squares that don't give a defense bonus while you camp on squares that do.  And of course be the one to attack first.

Remember, currently the best strat is to spread fast as hell placing cities near everything you see.  Early game the ai can build up a bit faster than you might expect ( it rushes like a zerg player).  You need to spread fast enough to counter the initial army spam.  Then you wont be challenged again till your next game.

it is most likely the case that your enemy ai had enough time to build grouped units - and grouped units are broken as hell (although apparently not as much as pioneers, organized, teleport, or arcane doom *rolls eyes*).

Currently, a unit which starts as a group (3, 4, 6, 8, 9, 12 fighters as 1 unit) retains it's entire attack and defensive strength no matter how much damage you do i=to it... so even if it's down to 1 guy out of the original 12, he still hits and defends like 12.

It makes them tremendously hard to kill, especially under the current combat hit/miss system  You need pretty much true-damage magic to make a dent in their hp (i've seen 120+ hp), and even that takes multiple turns when you had arcane doom + flame breath.  With flame breath alone... good luck.

I don't think this is true anymore.  In my last game every time a unit died in an enemy squad the attack and defense that was displayed dropped and my archers did more damage per hit.  Not really sure about attack since they never hit me and didn't lose any archers.  But the defense definitely dropped when a guy died.

Explore faster I'd say, you should atleast find your closest neighbours fairly fast. And then keep an eye on them. If you start getting negative multiplier, spam some squads of axe/shield men (or spearmen if you don't have metal) untill you are neutral again. Making units without armor doesn't cost that much and it does the trick till you can afford some more expensive units. the key is to start soon enough, before they are hostile to you.

Or if you are too late, you shouldn't have too much trouble taking them down with the correct spells and some archers if you have them. There's an air-spell that knocks back enemies while doing damage (can't remember the name). So you can easily avoid damage yourself by using deflect, hitting them a few times and then knock them back 2 squares so they have to run upto you again. It helps if you have a few more casters beside your sovereign though.

Yes, it sounds like you are fighting parties or squads of troups with your singletons. To be able to take these guys down you have to either use the techniques described above (reply 7) or make parties and squads of your own. It took me a bit to figure out how to do this myself so I feel your pain.

In the troop training dialog box you will notice some buttons towards the top and on the left side. The first icon is a single silouette, the other has three silouettes, and there are more. Research the tech "parties" under combat, the you will be able to push the button for parties. THis will make them MUCH more expensive but their stats will go way up as well.

Also the other thing I never noticed until recently is the buttons for adding experience to your troups. THose are the other buttons on that screen.