Survival and the PL
I have tried to shoot 0.1 only, but it didn't work very well, because if you have less energy than the other bot it is most likely that you lose anyway. But you give the other bot the chance to collect more bullet damage. If you have a weak opponent you have a good chance to hit him and get on some point more energy and a good chance to win, and if it is a strong bot you deny him the additional bullet damage.
I had played with the energy slope to detect when he might be at zero and adjusted my bullet power to this value. The energy slope was quite nice to react on bullet hits and other energy losses and worked very well but it turned out that you have to react on some battle situations where it is better to shoot normal otherwise the opponent get to much bullet score.