It doesn´t really matter which units you use. An all level 1 unit army, a only close fighter unit army, only 1 single strong unit expect of all 5 units, no rage and/or no magic - all is working and not really hard. The game is way to easy.
I played Heroes 3 too (over 500 hours). The main different from heroes to kings bounty is the unit system. Every KB unit has skills that are good for hundreds of different strategies. If you use this skills well and if you look at the skills of your enemies the game is easy.
If you rush into battle and use your units to attack the enemy as fast as possible (the normal heroes way) without checking the turnorder/reaching/skills from the following units/enemies the game would be hard.
My advice: forget every heroes "the more the better" tactic and compare every single unit. Believe me, that is the best part of the game.
