So sorry you had to experience that.

It's been my experience in other games as well as xiv that typically the player who feels the need to brag and boast about how great his gear is as soon as the party is formed, will most probably be the player making the worst mistakes. By endgame he should have had not just the gear, but the experience and job-based knowledge necessary to compensate for an undergeared/inexperienced tank as long as the tank is putting effort into tanking properly, which I'm assuming you were doing since you went on to win it with a party of other less experienced players. (Congrats on that, btw~) You did good. Don't let him get you too far down.