The sloping ramps in this dungeon are your allies. Tritons will repeatedly slide back down these ramps in turn-based mode. They may or may not slide down each time they are hit depending on the speed of your system. You can stand at the top of a ramp and let the Tritons come to you, peppering them in turn-based mode with your bows until they drop.
If this seems too much like shooting fish in a barrel, you can toss arrows in real time until one of them gets too close. Then press Enter to get to turn-based mode and the Tritons will slide down the ramps waiting for their turn. Usually. ;-)