Alternate: it is doable with with an ultralight. Build the Schneider mech out of the shop, STOCK PULSE SWORD and your choice of main firearm. First phase STAY ON HIM. In fact, right as the fight starts, boost underneath him, the first volley of missiles will shatter on his own shield and you'll take no damage, plenty of time to get a charge swing of your sword in. When he's not firing salvos of missiles this guy likes to do a couple pirouettes over your head then take off in the opposite direction you are facing, don't get confused and STAY ON HIM. if you can keep close, he won't get a good angle to fire his main cannon, and the pulse sword chunks his shield. If you have two bays of the STARTER MISSILES equipped to the shoulders you can sometimes crack his shield before Balteus fires his second salvo. KEEP HIM STAGGERED, you can get him to phase two without a second shield sometimes if you are aggro enough (the SHOTGUN can really help with this, but you might find phase two to be a bit ridiculous with that short-range of a build)
PHASE TWO: DONT GET HIT. you are a freaking mosquito and Balteus just broke out the blowtorches for your annoying ass. Between his main cannon, 2x the missiles per salvo, and those dang torches, if you catch stagger you are gonna have a bad time. Keep max distance, always hold some stamina in reserve, and when he swings for you (sometimes one big one, sometimes a combo, watch for it) dodge to his backside and get a swing in, the largest attack opportunity is after he does this biiiig area denial attack where he hovers in place and sprays fire everywhere, get good hits in on his shield here. Shield is popped and you are back to phase one tactics, pressure that damage, only watch out for a couple really sneaky things this guy does (point blank main cannon, boost away with the engines right in your face OW OUCH). You won't be able to get him in one shield pop after phase two (I'm sure it's possible but I couldn't) so get good damage in then reset.
This is definitely not the easy way, but it sure is fun