The greatest damage can be done by any energy weapon once it drains energy.
A phys mech requires at least 2 heat modules and 3 energy modules (to resist 2 turns, the third one runs out of energy) + resistance module, which leaves only 2 slots to place HP plates.
Phys is vulnerable to both heat and energy and his only defense is a high HP and the strength of the damage. What is happening now is that, for a phys to achieve a high HP value, even with platinum, it must sacrifice either heat or energy and that isn´t good.
If you don´t have platinum, then it´s much more complicated.