Easy mode Caster Kill...

Stun @ 80% (roughly 5-6 sec)
2nd stun 3 sec
3rd stun 1 sec
WHM Fluid Aura his next action.

If you're party failed to kill it by now... I forsee you having trouble with spumes anyway.

If any OT wants to do this differently.

A.) His/her method should be successful.
B.) If his/her method is unsuccessful he/she should change it.
C.) He/She should be replaced.

It's OK to try different things... but if they don't work... it's OK to say... "welp worth a try" and go back to what works.