Well, my favorite army consists of one tank, 2 damage dealers (preferably 1 of those is a backup tank), and 2 support units.
A tank is someone with 20% or more physical resistance (because most of the enemy troops inflict physical damage). Damage dealers can be both close combat or ranged units, and support units are the ones that can resurrect your troops or boost your tank's defenses (Like the Archmage).
It will be very hard to give you a strategy, as everything in the game is random and there's no telling what you can expect.
This is why I offer you to try a game on easy difficulty and learn the game through. Experiment with various lineups and you will be fine.
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