Well, you're on the wrong page (you should check the PAX page), but as long as we're all here... First, have you upgraded your weapon to Mk II? If not, cut the arms off of the RHINO enemies in the first level until you have enough tungsten to do so. Next, it's actually easier to break his leg armor off if you don't lock onto his legs - just stay under him and beat on those legs until the armor pops off, wait until he's got a leg up in the air to stomp you, and then hit the leg he's standing on (while the other is in the air) to knock him down. Once he's down, lock onto his face and get as many good hits in as you can. He'll stand back up after a bit, and start trying to stomp you and shooting missiles at you - avoid the missiles and knock his leg out again and beat on his face again, and you'll take him down. If you stay at range when he does the missile attack you'll likely get the v2 version of his weapon, which is a monster and well worth the extra effort.