Getting tired of this certain player Harassment

[Niwashi]

Blacklist doesn't work in duties however

SE really needs to fix that.

[Orophin]

If it makes you feel better, Leviathan Ex is a pretty cruddy fight for a MNK if the other DPS aren't helping push phases. Everytime Leviathan jumps, apart when spumes are up, you're losing your GL stacks which is a huge dps loss.

As others have said, if that person keeps pestering you or slinging your name through the mud, report them.

[Buff_Archer]

Here's what I'd recommend doing in that situation, if it appears that they didn't queue into the dungeon with the other two party members (even if you used roulette the rest might not have). Pretend that you don't know/remember them from before and try to phrase things in a way that makes everything they say look that much worse. Like if they criticize the way you're playing say "I'm really trying to improve and I'm always open to advice and suggestions, I just don't see why you have to be so mean about it I'm doing my best." That kind of thing, you don't want to overplay it, but you can be nice without being sincere if your end game is to have this backfire on them. And if they keep it up just do like a or :'( or whatever and leave it at that and keep playing till you get to the point where you can vote dismiss them for harassment. If it passes and the other two players are obviously on your side, you've now potentially got two other people to report them (or even better, up to six if a Full Party) and it wouldn't hurt to ask them to do so. They don't need the whole backstory if what was said in party chat alone was enough to speak for itself. And I would be 100 percent honest in the GM report and say "This is a player who was abusive to me in the past and I thought if I could just pretend I didn't remember them they'd leave me alone and we could finish the dungeon and go our separate ways but they persisted in...".

A lot of abusive people are seeking a form of control over other people (you even mentioned thinking of changing servers- why should you have to give up your own friends and home?) so by orchestrating things so that what's said in party chat paints a picture of an aggressive player preying on the vulnerable (which you'd only need to pretend to be to further your own interests), you'd ultimately be taking control of the situation and could better use it to your advantage against them.

BTW you might know this already, but changing worlds won't prevent the possibility of queueing with this person, only data centers which really limits your options; perhaps that's what you meant. But that's not something you should have to do anyway.