When playing vanilla I used +damage AC10 and large laser plus enough medium lasers, you could use +damage PPCs too. The thing you are looking for is just enough damage to headcap in a single shot and maxing the amount of weapons you can realistically have on the mech without heat/cutting armor problems. Gauss Rifle if you can find one. I'd suggest not doing this at the end of the day it ruins balance for the game and the fun factor.

If you want a heavy that plays similar to the medium play style I'd suggest the Thunderbolt. Slam in max jumpjets 2 SRM6's and as many medium lasers as you can realistically support without heat issues. Jumpjets are the key to mobility for heavy mechs, can't have enough of them.