The gameplay is pretty simple actually, you just need overtake level your enemy. Focus levelling some of the unit especially spearman and mages. However beware that if your fighting at high level , this would make enemy gain more experience since they are fighting stronger enemy.
Example,
Level 15 your commander vs Level 1 enemy commander , the enemy will gain high experience from each encounter when fighting so end them quickly if you don't want them to bite your low level units.
Unless you have proper resources gain to recruit demon , make plenty of spearmen and archer with mage. They are like core soldiers for your forces.
<Demon or Spearman / < Infantry or Horsegirl or Beastgirl / Archer and Wizard. You wanna play this formation for 1st round then change formation if some of your troop die or they bring out specialized unit.
Horsegirl and Beastgirl are specialized unit to lay waste on Infantry or low level unit.
Demon are 1 range fucker that kill anyone infront of her except mages because magic.
Archer are the quickest by class comparison to kill any ranged unit so use them early on to kill the range unit especially mage to prevent mages magic away your demon.
Spearman are your tankiest unit to soak the damage.
Just play demon(expensive) or spearman in your frontline (1st colum) and archer as well mage in last colum and you basically can end the game
Don't worry about loss of your unit, they can be revived at main base. It is just expensive to revive them.