My best tactical decision is my defensive formation for fighting.

If I have slingers (Which I get as mercenaries sometimes) they are infront.
Peltasts follow, (If no mercenary slingers Peltasts go infront in front)
Followed by Archers
Followed by infantry.
With cavalry on the side.

Starts the battle with your army either at the highest point, and as far away from the attacker as possible.

Yesterday I was beseiging Citra, there was a full stack of numidians inside, and there was a full stacked numidian army outside led by the Faction Heir.

It attacked me (The faction heirs army) and I crushed both armies. the missile troops fired, as they charged with infantry and cavalry my missile troops retreated behind my infantry, and my cavalry ran around.

That basic tactic is the best.