Mine would be the Malboros cuz of their BAD BREATH... and Behemoths... cuz they cast Meteor on you when it dies which COULD wipe out your entire party... hey, you never know.

and as for SYG, Tonberry shouldnt be much of a problem unless you cant kill them fast enough but late in the game... you would beat him EASILY by just summoning Diablos like 3 times... then he should be dead... make sure the character equiped with the GF is very friendly with the GF so that it summons faster and well as for the KING Tonberry, lol he wasnt much of a problem either... yes you cant use Diablos this time but wut you could do is to use Aura(or wait until your character is low HP and use Hero on him/her) and keep hammering away with Squall's limit breaks and let the rest of the party members heal(or if you waited for Squall to get to low HP, just keep casting Hero on each Character per turn but Squall just keep hammer away). One Lion Heart should kill him.