Guild Wars 2

I Would like to know which Weapon sets are the best for leveling and Tagging mobs?

Also Is there a certain build to run?

I have been told mesmer is quite difficult to level and currently am at level 40.

Congratulations, you made it over the hump. Level 40 is a big milestone because you can grab Deceptive Evasion, which makes the leveling process so much easier. As for the build and weapon sets;

https://forum-en.gw2archive.eu/forum/professions/mesmer/Lazy-Kai-s-Guide-to-PvE-Leveling-Guide-Update (second post)

TL:DR – GS + Sword/Focus, 20/20/0/0/30 Shatter build.