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Simas. I haven't played in a long time, so I'll talk about the general basics from memory.
Your army should have high-initiative troops (for example, artillery) so that your hero has time to cast a useful spell at the very beginning of the game, and also to use a spirit (convenient at the end of the game = Reaper) before the very first enemy unit goes. You can also upgrade your hero so that he always has more initiative for all his units in the first round of battle (forgot the name of the skill... it seems to be for the Warrior).
Regardless of the losses of your troops in the middle of the battle, they can always be resurrected at the very end of the game by 100% if you have the resurrection spell (all units - up to the fourth level), plus - I always kept Inquisitors in my army so that they could resurrect fallen units (even level 5), and also increase rage points during the battle. Usually (at the end of the battle) I left one unit (1 or 2 units) with the enemy... usually a weak archer or a slow infantry unit. I deliberately didn't kill it... but I upgraded spirits (summoned them many times, 5-7 at a time, and different ones) and restored all my troops that I had lost in the middle of the battle... and I tried to make sure that I came out of the battle with almost maximum mana and rage points.
I also liked to keep Red Draconians in my army - so that in the first round they could hypnotize the enemy archer or mage... besides, they have high initiative.
I also liked Shamans - in the middle of the battle, when I left the last enemy unit (weak, as I wrote earlier), I blocked it with Lina's orb (without attack)... and surrounded it from all other sides with shaman staffs (which heal their units) - and the enemy could not get out of the trap... and did not prevent me from restoring the lost troops in the end.
At the end of the game, I also kept dragons in my army (but there were still Inquisitors and Shamans).
There are many combat tactics, but it is desirable to disable the enemy archers and mages in the first round of the battle.