you can also try

Mess Spell, Slowdown, Poison, Fiery Attack on first mob
then poke the second mob.
When I manual burst with my chars I prefer killing(or poking) the second mob than using some more SP to burst on it because I think the exp is still same or if there is a difference it's only a tiny difference.
And that also saves SP.

But if you are using IM remote, then you can use
Mess Spell, Fiery Attack on pet, Slowdown, Poison, Poison
it saves SP than using mess spell on last round.

Just make sure that you're slower than the water but not the same spd (like spd 0) as the fire attackers.

And also you should have your skills MAXED to get the most out of bursting.
If a fire is assist bursting, it can get as big exp as what water gets during burst. Fire Assist Burst is just like bursting an earth or a wind.

XD