After you master the mech, get an lrm5 cooldown, and take into account the quirks this mech can pull off a perfect rainbow with only 5 lrm5s. This means it that on chainfire the lrm5s fire nonstop, one shot every .5 seconds for a constant stream. Finally the rainbow mech is not powerless against ECM. You can narc your own targets and then make it rain. Artemis does not stack with narc, so there is no need for it. You can use the extra tonnage to ensure you don't run out of missiles (which is your only weapon). Catapults heads are a bit easy to hit, 16 armor there will protect you if you catch a dual gauss to the face.
Did they change artemis stacking rules? Tag and Narc do not stack, but artemis stacks with either of them. This is according to my LRM test here: https://www.mechspecs.com/threads/system-test-lrms.8298/ That said, Artemis isn't really needed on LRM5s.