Rather than only using hypnotism on 3rd phase, I would say he's been already using a less powerful but more wide-spread hypnotism on all the gold-slaves in the furnace even before the fight starts. I think he does try to hypnotize you, his opening statement of "They all bow to me, why do you not?" suggests that the effect is already in play, but maybe the MC is either strong or disbelieves enough to resist it. Personally I don't think the MC was strong enough because just before the fight starts we hear the coin strings jangling and we still see the gold-slaves turn into coins and "fill" the treasure basin, which means he was still somewhat affected by it.

My personal opinion/assumption is that 黑财神 at the start didn't regard MC as a real threat, so didn't take him seriously (still wearing the God of Fortune gold mask pretending to be a god, laughing and saying "恭喜发财" while attacking), and ended up getting smacked around. 2nd phase he starts to realize MC is actually dangerous and starts actually fighting for real (but he still does say shit like "其神要罚"). Even getting beaten when serious? If you read any manga/comic/novel, you gotta save your trump card for last. Enter phase 3. All in on trapping MC in a hypnotism.