How about an Alpha strike?

For those not familiar, an Alpha Strike is when you fire of ALL your weapons in one turn. The AI computes how much heat and energy that'll cost and apply it. If you end up with a 300% overheated and/or -200% energy mech then so be it.

Variants could be
Alpha Missle - All missles fire off
Alpha Energy - All energy weapons fire off
Alpha Heat - All heat weapons fire off
Alpha Physical - All physical weapons fire off

This option is only available as an opening move. Hence, an alpha strike.

There would have to be a drawback to using it, otherwise people would just have weapons of one type, and use it to do really heavy damage to their opponent in one go [possibly even win in one hit].

And at the higher levels, where mechs have 6 weapons, this basically means you could fire 6 weapons in one go. That seems highly unbalanced, so I'd have to vote no on this.

Alpha strikes are a bad idea IMO because you could easily build mechs which can obliterate another mech in a single shot on any level.

The game is balanced around the fact that you have a limited number of actions. If you have a mech torso which support 6 weapons and all fire at once you effectively get 6 action points.
This would kill almost any mech especially with not shields up.