Rework the enemy targeting algorithms
enemies always seem to focus fire on the nearest player, which is especially troublesome for a couple reasons.
1. One member of the party dying ends the mission for the rest
2. Most classes don't have ranged attacks, so it's not unlikely to get a whole party without any ability to shoot back.
Some way to influence enemy targeting such as a mark or aggro mechanic would go a long way into making challenges less prone to spiraling out of control in a single round.
Technically speaking, doing an "alpha" on mobs or party members IS the best tactic, as 4 half wounded wounded things are more dangerous then 3 healthy things.
If anything, maybe players in line of sight could act as partial cover, or some applications of skills could cause rage effects that make those mobs want to target the character responsible, and that could be one aggro mechanic.