H'El sends a creature called the Eradicator to stop Superboy. Superboy defeats him connecting with the city and manipulating it to impale the Eradicator with spikes. However, the Eradicator is not out yet, as he throws a metal shard at Superboy's back; piercing his skin because of the red sun weakening him. Despite this, Superboy pulls it out, sends it back at the Eradicator, and kicks him into a temporal portal that sends him somewhere in Smallville.
Supergirl wipes the floor with a bunch of feral clones. She may not be as experienced as her cousin and the red sun is sapping her powers, but she made those heavily armed clones look like wimps.
Clone: I cut pretty throat now. (holds spear at Supergirl's throat) Supergirl: (melts blade with heat vision) Cut? With what?
Supergirl vs. H'El. He uses a shard of kryptonite to get the upper hand, but she shows that she not to be underestimated when she strikes him under the nose; giving her time to pick up an axe to fight.
Supergirl: My powers may be fading under the red sun, but I am not powerless.
H'El somehow manages to send himself to Superboy's time period to attack him and Supergirl at the same time. The two timelines synching up means that Superboy and Supergirl can both hurt H'El at the same time.
In the finale, Superboy pushes the entire city of Argo to give it the lift it needs to escape Krypton's atmosphere as history dictates it. The effort used up all of his strength and he dies in the wake of Krypton's destruction.