Gass Hole: Leonardo's traveling companion, in this episode at least
Heel Realization: The entire point of this episode is for Leonardo to see what he's become and to heal. The final fight (where he realizes he's fighting his own rage and guilt) is the moment where it finally dawns on him.
My God, What Have I Done?: Leonardo has an example of this when he loses control and injures Master Splinter. Although he doesn't actually say the line, the look on his face after it happens and his subsequent remorse show he was clearly shocked, and deeply regretted his actions.
The Only One: Played for Drama. Flashbacks show the others (reeling from fighting with Shredder) barraging Leo with pleas for help and to come up with a plan.
Donatello: Help me. I'm hurt. Michelangelo: We're not gonna make it. Leo, help me! Raphael: Leo, the Shredder's comin' for us! You gotta do somethin'! Splinter: Save us, Leonardo! We are all counting on you, my son.
Scenery Porn: The Hidden Land, whose background features an art shift, to boot.