I would either power up a shot with more energy than I have and probably try to hit the oponent with it...
But you can also just spend the enrgy on growing body, slime and shell like crazy....
But in the end I would probably rather try to shrink to 1 in body size as fast as possible (Check the torpedoe behavior from spinner), and once you're this small you can either try to run away, fight with ties, or make lots of shell and stay put, only moving if you get hit....
Either way the oponent should spend more than it gains trying to kill you...