Depends on the level of PIS/CIS involved. But if all of Team Flash is onboard, and we aren't putting the Dunce hat on Barry, they should be able to take out Iron Monger.
Given the crap that Cisco can build, he could probably pull something out of his a$$ that temporarily disabled the suit, giving Flash time to detach a few bits (like Tony did in the film, forcing him to open the suit and expose himself). And once he is exposed, it's game over.
Edit: Actually, just send Caitlin. We saw how poorly that suit handles extreme cold. 😛
Tony jumped on his back and detached some of the targeting system wires, forcing him to open up the suit, so that he could see. It's not like he was just pulling pieces off. He was getting his ass kicked up until that point. Plus, Barry has used phasing to destroy machinery before, like in the episode where RF captures that one scientist to help him get home. Barry phases his hand into the one panel, damaging it and pulling some bits out. At about 1:20, here:
He also used phasing/vibration to split metal cuffs during the s2 finale. At around 2:48:
So, he has potential options. And Barry could distract him, so that Caitlin could try to get close. I agree that she would get absolutely murdered if she pitched up alone. My initial comment about sending her was meant more tongue in cheek.
Was just thinking, but another thing to consider is Cisco's almost superhuman ability to build virtually anything the team requires and, in this case, doing something that could do permanent damage to the suit. I wouldn't be surprised if he could build a mini-EMP or something, if necessary. He's made more ridiculous tech than that before, while on the show. Ultimately, a single big mech suit is a lot more limited in options than the entire Team Flash is. Not to mention Stane was not exactly a combat tactics master.