Me (Rune Knight or Knight class) - Imp
Vivi - Mage (squid face from IV)
Terra - bomb

Tactic: me and terra make out, while vivi looks akward. Mage and bomb make a light show and blowing itself up. imp will stumble around and attempt to attack the opposing characters feet. at that time which the opposing people are laughing at us as a party. Terra after pulling her tongue out of my mouth would cast silence on them to make them unable to cast a spell. Vivi would then enchant my blade with flare and i would finish off the weakest melee characters first leaving the strong one by themselves.

Forest tactic: Imp hides in a tree, mage casts invisble on bomb, bomb acts as a land mine. i make out with terra, vivi stands there looking akward. I charge in first using runic to stop the first spell from being cast. once used terra enacts silence on the strongest spell caster. bomb explodes on the strongest melee attacker on the opposing side. when in close combat with the strongest melee character, I will call imp to come down and attack the melee agent from behind.

Dealing with other summons:
Ifrit - Weak to magical attacks but strong elemental damage. best be countered with paralyze from Mage, and let him try to do magical attacks which can be absorbed by runic.
Shiva - Silence, she is neutered. ill send vivi, terra and bomb after it and she is done in a few seconds.
Ramuh - bit more tricky as i would have to use myself as a lightning rod litterally. silence is the best bet so that way not to get paralyzed and ranged attacks are important so terra will have to morph and blast his ass.
Odin - have razor wire ready to cut up his horses legs and take him on in melee combat. he is nothing without his horse.
Ark - vivi and terra will generate an emp on my call to take out that oversized transformer.
Phoenix - Shotgun is how you kill a bird, and cast ice on its corpse to make sure it does not get back up.
Eden - same thing for ark, emp will kill that sucker dead. be dead faster than the machiraina.
Bahamut - now this is where all my training and experience with dragons comes in from dungeons and dragons. i throw a sack of gold coins at my enemies or make them look shiny. bahamut takes them away an makes a nest out of them.
Diablos - apparently his weakness is empty bottles, have terra cast blind on him and stick him in a bottle and put a cork on it.
Tonberry - i just say to it as it is summoned that those who summoned him killed his many of his kind. *doink!*