Well i've been killing this guy for quite some time and i can assure you that having mages in 2 spots is the best way, altought your strategy could be a good one, well the strategy that we've been using is to have healers and casters on both sides, so if a crack spawns in one place the other healer can cast without trouble.

After hellfire theres gonna be a lot of cracks i think six or something close, well if you have people spread too much the cracks will spawn everywhere, in the strategy we use the cracks may appear at the back of ifrit with melees making a corridor to the back so u can skip it without involving mages, the other way that those cracks appear is in both sides of the area (where healers and mages are) so melees can stay put doing dmg and healers and casters can eveade those cracks quicker and have lesser cracks on their sides making the run shorter so they can go back and cure the tank.

The most important thing is after hellfire keep your tank fully healed all the times so when these cracks appear he has enough hp to stay alive while your healers get back to their spot.

Well about plumes with this strategy only melees have to evade those if they're not close to the tank, mages get a safe spot all the time so your caster and healers should only focus on cracks.

But feel free to do it the way you feel like it.