Catapult Prime. An effective if not uber mech in 0.3, the nerf to I.S. LRMs really hurt it, along with the Clans getting Heavy Lasers; which can fire before the Catapult's medium laser can fire. It constantly runs low on ammo, the armor is so-so, it's slow as shit, and it's also more expensive than the Arrowpult, which makes little sense. The Bushwacker with 2xLRM15s and 2xERLBLs is a better choice most of the time.
A-Make it cheaper.
B-Upgrade the medium lasers to ER medium lasers, and ditch the dumb ELRM-15s for LRM-20s.