They recently changed flash step to be a PVE even off cooldown, it wont show the just like the evasion roll but it does work in PvE, you just need a shard to do it if its on cooldown.
Only the large circular rocks can KD you, FG wont work against those you have to , all of the other circles/ground effects are just damage areas you can ignore they dont CC or even do much damage if anything I was still moving around too much. You cant afford to really get knocked down or staggered more than once, but also cant afford to waste too much time dodging everything. I was also having some packet loss so that might also explain some weird lag. But generally the last phase is easy if you have the damage to kill him, you only have to the large circles literally ignore everything else and just go nuts.