Remove global cooldown on field AoE and Wall of Force. Large scale fights usually revolve around a full melee push from one side which ends up being unstoppably overwhelming. This is how every siege I've spectated turns out and small scale fights often go the same way.

Since DF1, WoF was triple nerfed:

1. Mana cost increased to 50
2. Global Cooldown linked to Field AoE
3. Cooldown increased from 30sec to 60 sec

These aren't just nerfs, these are extreme changes (5x mana cost, double cd duration). Often a group fight can be decided by counting how many destros your opponent currently has online.