Use level 4 and 5 units if possible (though early on use inquisitors), get the Royal Thorns ASAP. Ranged creatures + one support unit (Archmages are nice for magic shield + they can do massive ranged damage when in Trance), 1 or 2 resurrectors (Inquisitors early then highmages, Paladins) and your tank who would ideally be a dragon
, but you may have to use Royal Griffons or something instead... This is a good system.
On Dragon, get treasure search and Crushing blow ASAP and keep them balanced so you will have enough rage to use them.
If you really must then kite (run around dangerous monsters) to thieve treasure and flags.
In Mage tree aim for Order and Distortion as soon as possible, keep an eye out for phantom, heal, stoneskin and resurrection.
That's just a little general advice, it's hard to know how to advise because you are vague on specifics. Every battle has a way in the game (though in general many find the first island one of the most challenging phases of the game).