Cortana moves quickly, putting her fast (as much as possible without being superhuman) reflexes to what she judges as their best possible use: doing this in a way that minimizes further harm to her friends.

She's tired of heroic sacrifices.

She doesn't want to use Elidibus's damage redirect more than she has to. Her pistol was just about out of bullets before they entered this fight, but she whips it out and fires the last bullet in a running shot at one of the snakes guarding Steel before throwing the empty gun at another. She flings herself at the bier on which Steel rests, attempting to tackle/crush/send flying everything with the force created by her density and speed.