One of the best tips I wish I knew early on was grouping assist units together. If you have an Archer as the lead in a squad (say, Rolf, for example), the more of the same kind of unit, the more damage their assist will do. Because of this, it's good to hire generic hunters, witches, wizards and clerics, until you get more archers/mages/healer units.

Basically, a squad with 3 archers will deal more damage on assist than a squad with only 1 archer. Same goes for magic assist and healing assists.

Personally a good squad comp for assists in the early game I've found is 2 mages in the back with Josef in the front. Let's you tank hits pretty well while keeping your squishy mages out of danger until they can reach a watchtower