mushin's tower floor 7

[Omaii]

So I've been trying to beat the boss on F7 so many times but failed. After I get his HP down to around 30k he starts resisting all the attacts. I understand that he might be resisting more than at the beginning, but he literally resists everything. My skill build is quite decent and I play as a Force master lvl 50 HML 2 . 

Does anyone have any tips because I literally tried researching it but nothing helped much.  At first I thought it was a glitch but I don't really believe it is. 

 

[Rappel]

you dps too slow and hit enrage time then he resist everything till you die

[Sandaion]

There's a time limit (i don't remember how long). When time's up, he'll come invincible, resist all damage and continue to attack you until you die.
I passed 7th floor by using FM burn spec. Check this skill set and some advise below: https://bnstree.com/FM/pub?build=50020002553225641381312653223833250323803210632251332353330021241322632123922237222431141012
As soon as you engage in battle, immediately use Glacial Beam (3) to stun and freeze him, then switch to fire stance and Inferno (X)
Use Short Fuse (C) to inflict burn then Meteor (V), keep spamming LMB RMB F to damage him and stack chill (3 chill -> freeze). Watch his animation carefully to predict what skill he's going to use and cc him, If he's frozen, Force Blast (1) to stun him, if you can't freeze him in time, use Glacial Beam (3) or double tab to cc him, don't forget Q E SS to dodge.
If burning status is over, use Short Fuse or stack 5 ember then Force Blast (1) to inflict Burn (whie target's burning, fire skill's damage is much higher). When he resists your attack (not after time's up), use frost armor to protect yourself, when he finishs attacking, unfreeze will make you dash back and freeze him, stun him and continue to attack.

Edit: forgot to tell that you can also use Phantom Grip to cc.