A mesmer basically is unkillable because of the magical 7 'defensive' (godmode) skills. It has a 3 second window (without counting dodges) to be hit every 30! seconds.
Add to that the ability to 1 shot basically anything.
Add that to there still existing high mobility teleports. (on low cd)
Add that to various gimmicks mesmers have (attacking while dodging) (clones in lower leagues that basically do free dps+let newer players zerg on a single point because they cant figure out the reall one)
Add that to easy bursts access.
Add that to a medium health pool.
And you have a class only specialised builds can deal with, the best part is how the mobility can most of the time make sure the 'counter' builds cant even catch up with them.
Boonbeast is too tanky can easily 2v1/3v1 by traiting for mass boons.
What trully needs to be done? Give mesmer portal back. increase cooldowns on atleast half of the defensive godmode skills (i am looking at you staff 2) decrease active uptime of defensives. (stealth)
Boonbeast nerf boonprocs, boon uptime.