besides using mods such as lusted's; a very simple trick you can do is these two things:

- triple the kings purse value in descr_strat.txt
- don't blitz the AI for the first 30 turns or so. during this time only attack rebel settlements and excommunicated factions.

this will not improve the common sense of the AI; but it gives him the time and funds to build up quite decent armies.
As hungary I ended up getting attacked by 2 full stacks of Russians (boyar's sons; cossack musketeers and dismounted dvor; NO peasants) at kiev; another 2 full stacks of russians at halych in the east; all this in the same turn mind you; three stacks of turks are on the way through sarkel;
and in the west i'm having to defend Vienna and Zagreb against uber agressive venetians and danes.