Using Diablos is very handy against regular Tonberries, but there comes a price... it is useless against Tonberry King, and as you don't have the chance to change junctions, this is awkward. I'd suggest having a spare GF on whoever has Diablos.

And the Meltdown-Aura-Limit Break combination, particularly with Squall or Irvine on Aura, is always recommended.