Reinforcements arrive to save the day. This is Grimlock, the leader of the Dinobots, with (clockwise from center, mostly obscured) Slag, Sludge and Swoop (the remaining Dinobot, Snarl, was omitted from the film except for a few frames). The Dinobots were a convenient convergence of characters from the comic books (which tracked vaguely with the TV series, but not always), and the emerging fascination amongst children with dinosaurs at about this point (or rather, the emerging marketing phenomenon of same.) The dinobot toys were quite well made, and the story that accompanies them in the comic series was a fairly creative one (which I won't repeat here, since it would just sound stupid in the retelling).
Optimus Prime, leader of the Autobots, leaps to courageous battle with Megatron ("Megatron must be stopped, whatever the cost," he announces). Optimus' character had a strong sense of honor, which probably accounts in part for why they killed off his character (honor is not a concept beloved of marketing departments). The battle between the two goes back and forth for a bit, till Optimus gains the Upper Hand, and is brought down by Sinister Trickery by Megatron. In the last frame Megatron is begging for mercy theatrically to gain time to reach a gun hidden behind that bumpy thing between them, with which he inflicts fatal damage upon Optimus. Hot Rod jumps on him to prevent this, which doesn't work, but distracts Optimus at a critical moment, allowing Megatron to shoot him with the nefarious pistol; this may be an "I killed Caesar" sort of reference.
Both leaders are more or less fatally wounded in the battle. Standing over Megatron in the lower frame are Starscream, Soundwave and one of the Constructicons (Decepticons who transform into construction vehicles, the first of the combining robots).
Soundwave was generally very loyal to Megatron -- after the grievously-wounded Megatron is abandoned by the other Decepticons, Soundwave carries him to the spaceship in which the Decepticons are leaving. Oddly, Soundwave takes the side of the Decepticons who favor throwing Megatron and the other wounded characters out of the ship to save weight a bit later on.
Here's Arcee again. Really, a robot with breasts, lipstick and Carrie-Fisher cinnamon-roll hair?cowardice, valor, diplomatic immunity