I tried holding back but no more!
Ok, Ogre Battle is all about managing your units. You have to make competent fighting groups that will be strong enough to win against most enemy units. That means you will have to build your fighting units slightly differently and opt for the best unit for given encounters. Ex: a unit with 3 archers would do very well against a resistant front line that hides 2 mages and a healer in the back. Of course you will need some special units for 'unique' tasks that are plentiful throughout the missions (VS undead units, fast flying units, all shield units, VS petrification golems unit, etc.)
Aside front pure battle strength, you have to consider the alignment/class combos of the units in the sense that you will require at the very least 1 evil, 1 neutral and 1 lawful unit. Near the end of the game, you will have to split said groups to have 1 pure evil, 1 half evil, 1 full lawful and 1 half lawful since the towns are very picky with the 'liberator's' alignment.
When you become more apt at the game, you can then start cheating the system to produce 'weird' units. Healers in general are high alignment units and as such cannot exist/advance in evil groups, but it is possible to create units and then later mix them in with others to remedy this.
Now, where the real fun starts is that you actually have to keep your units the weakest possible if you aim for a high Chaos Frame. Meaning you must kill the least possible and generally work with very few exp (being 'stronger' than the enemy makes you the 'evil' side of the war). That few exp will also have to be split between your 10+ units. At this point, it's not uncommon to have town liberating units that never receive any experience.
Although you'd think keeping your Chaos Frame the highest possible would be a good idea, effectively it is not since certain heroes won't join you if you don't match their 'alignment'. As such, at certain points of the game, you may want to take a Chaos Frame dip for a certain hero only to bring it back up later for the best ending.