In the setup of the battle, try to anchor your flank to an obstacle, such as a river, so you only have to focus on the other side of your army. If possible, try to place your units on the high ground, as archers shooting from a higher elevation have better accuracy and further range. Units fighting downhill have the advantage too, as it is harder for the enemy to attack uphill.
The best practice when setting up your army as a High Elf is to place rows of armoured spearmen in the frontline, which protected skilled archers and powerful mages further back from the enemy. This allows the archers and mages to damage enemy units from a long range, while the spearmen protect them from cavalry charges and hold a strong defensive barrier against attack. It’s also recommended to have some cavalry behind the front line just in case any enemy units do manage to break through, or attempt to flank you from the side.
It’s best to place your slowest troops at the front of your army and the fastest at the back, allowing them to come into the fight at speed when needed. Remember to keep units in reserve, usually your best, so they can swoop in when the enemy is weakened. No use wasting their health on the low-tier frontline of an enemy!