You can unlock this achievement during the very first mission - specifically when the droids attack you on the Trade Federation battleship. Just keep attacking them without stopping, and you should be able to reach a 30x combo multiplier.
Another great mission where I easily got an even higher multiplier is the one where you fight Darth Maul, called “Better Call Maul.” You can reach the multiplier during the first wave of droids, right after Maul runs away from the first fight. Attack the droid squads, and right next to them you’ll see a group of droids playing instruments. Attack them as well to increase your combo. You can do this by throwing your lightsaber at them.