Zilla Jr starts moving towards Amblygons location, and Amblygon starts moving to Zilla Jr too. They see eachother swimming towards eachother. They both speed up to collision. When they collide, they are at a stalemate.
Zilla Jr lands the first punch on Amblygon, then bites Amblygons arm. Amblygon screeches out in pain, but is cut out by a tail whip to his stomach, he bends down, and is hit on the back.
Amblygon gets up and sprays his acidic breath at Zilla Jr, Zilla Jr moves out of the was but is punched by Amblygon. He feels pain but shakes it off and drags Amblygon to the surface.
When he gets to the surface, Amblygon notices his spines flashing, and tries to move out of the way but is punched and held by Zilla Jr, and is engulfed by Zilla Jrs beam.
He punches Zilla Jr in the face, sprays his acidic breath and swims as fast as he can away. He is severely burnt on his chest. Zilla Jr cannot pursue and that last acidic breath burnt his left shoulder badly, and it is bleeding.
Sorry if this is a bit short.
Also this is how Zilla Jr's atomic breath looks.