Advanced electronics SRM/LRMs. Capable of being programmed at launch for either Swarm, Narc, Artemis, Home-on-Jam, or other electronics packages (but not warhead or missile modifications), they allow for one ton of ammo to serve the purpose of many.
Enemy ECM bubble? Toss off a volley of Home-on-Jam, so their ECM acts as a targeting and cluster bonus.
Lots of small guys clustered together? Send a Swarm in, and let the missiles have fun.
Someone else has Narced the opponent? Your Mech loads the frequencies into the missiles, and they all hit.
Spike Gauss rounds (or Autocannon rounds). Take the Gauss damage, divide it by 2 (FRU). Apply half that remaining damage to the armor, and if the remaining armor is less than 5* the remaining damage, apply the remaining damage (FRU) to the internal structure, and roll on the Critical Hits table (max of one critical hit from this though). If the remaining armor is 5* or more, apply the damage to the armor as normal. If there is no armor, apply the halved damage to the internal structure, and the rest is lost.
I.e. Gauss Rifle: 15 pts
Spike: 8 pts
Half to outer armor: 4 pts
Armor threshold: 20 pts (if remaining armor is less, apply remaining 4 pts to internal structure and roll for max of one critical hit. If remaining armor is equal or more, apply to armor.)
No point is doing this for weapons doing 3 pts or less of damage.
Tactics: If enemy target is heavily armored, toss regular shots. Once the armor is down to ~20 pts, toss spikes to crit it to death. If armor is gone, toss regular shots.