Another solution for pve imbalance.

Give bosses multiple points to hit.

There is another mmo, named Phantasy Star Universe. It used to be just like this game that melee users were gods of the game since they had an axe skill that let them hit 10 targets at once (5x2 in one hit). The magic users were left out since their magic could only hit one target at a time. Bosses in the game also had multiple points to attack (ie arms, head, legs, etc) that comprised the same hp pool. As a result, melee users could kill bosses in seconds and magic users took almost hours to do the same thing. There were also gunners, but their shotgun could fire 5 bullets at a time, so that doesn't apply to here.

Eventually they applied a patch that let magic users attack multiple points, and even an extra where they could time their attacks to do extra damage.

So if bosses here had multiple hit "points", magic users could be useful again since their aoe can be almost as effective as 5 aps. Axes would be useful again other than using heaven's flame. Barbs could retain aggro easier since they're aggroing multiple times from the same boss.

I know, can't make as much money from these interval people anymore if you did that. But just a suggestion to pve balance is all. Not like 5 aps is going to stop being broken. Casters still have to channel and cast each aoe, wait for cooldowns, build up chi slowly, etc. It'll just give them a more competitive edge. Maybe big bosses could have 5-6 points, while the smaller ones are better to do with aps users.