Man, futuristic power armor is too cool. I made a post about bashing damage being the best bet, but then remembered that power armor is very heavy. So not only will impact force spread over the armor surface, it would be tough to knock the target backwards or even over. Maaaybe a couched lance.
And obviously slashing will only work at all if there are, for some reason exposed bits of sensitive flesh or mechanics. And there doesn't need to be, even medieval full plate had armor hang over the joints. This is like that except encumbrance is far less of an issue.
So I guess piercing would be the best bet... But then you're going up against advanced alloys with mere bronze or steel. Hopefully high calibre bullets, which should work relatively well.
Then there's alternate possibilities they might not have planned for:
Ink over the oculars. Hope they instinctively smack themselves in the face.
Soft ground, use that weight against them. Or if they aren't heavy, bashing becomes practical again
Lots of fire. Without a venting system, they'll eventually cook. High specific heat could make that take a sustained effort, though.
Ballista/cannons to overcome their mass advantage
Trigger an avalanche or mudslide, preferably to shove them over a convenient cliff
land war in russia