A nice tactic is to have royal thorns. The AI is a sucker for summoned weak units and loves to attack them. And besides, if there are any dead units around, the summoned thorns can summon even more thorns, and that's a landslide
Your units multiply in no time. This minimizes the damage your main units take. However, in some games you may be unlucky enough and never get any thorns in the basic areas (I have that problem now).
Another option is to remember where the battle area was favorable. I fought like 10 battles in the swamp (one vs. Eghirra which was listed as overpowering) on the same spot where there is only 1 hex that the AI could occupy to hit my horsemen (are is 3 hexes wide). It was fun - destroying the hunters she had one by one, ressing the units that died, and then I had a full army vs. 1000+ peasants. With some magic spring, there were no problems defeating that army without any casualties.
That's for example