First, save before every battle, if you loose, re-start. Or be very careful with injury for Monsters. It can be very annoying.
You start with some hearts, and you will gain 1 more every 10 sec. Use hearts on Monsters for a special power. For exemple if i remember correctly for 4 hearts the eye will corrupt an ennemy After 2 hits, and he will fight along your side... Every powers are explicated. After it's classic; tank, support, long-range attack, etc. In your army, the first line will be depployed with the Monster below him (2nd Line) so try tanks in first position and others below.
And every Monsters have a Time counter before depployment, so don't make a duo who need too much Time, you will loose before they go to the battle. Only on one line you have to reach the over side, so even if your army on the top Line is destroyed, if you are faster than thé ennemy on the bottom you won.
I can't help more, and maybe there's some mistakes, i played it a Long Time ago :)