Best character to use would be Artiguana with at least level 2 upgrades. This can easily be done in 'That Was Then...', you will come to an in door market area midway through the level, where you will need to use Wild Mutt. In these area there will be an infinite spawn of Ant enemies, as long as you do not complete the main objective. Switch to Artiguna and use his , Frozen Slam combo, this is cause the enemies to be launched into the air. Rinse and repeat until you have kill 30 enemies with this combo.
You can also use Gravattacks , Double Smash combo. Must have at least level 2 upgrade to be able to preform this move.