I have never faced a boon prot monk - because I don't play PvP.
However, could you bring Shatter Enchantment or Drain Enchantment or Inspired enchantment to remove the usefulness of a boon prot? Inspired enchantment might be better to bring as it is spamable (1 second cast, instant recharge).
Conjure Phantasm is good when you go against someone under the protective bond or life barrier or protective spirit enchantment. It goes through those and eats away at your foe's life.
As for secondary profession for PvE, Necromancer and Rangers are good choices. The mesmer can spread conditions and create damage from both instilling conditions and even when they are cured. Necromancers do well with healing themselves and regaining energy (elite - Offering Of Blood). Since you are a primary mesmer, you won't get the benefits of Soul Reaping.
Necromancers and Rangers both have some good healing and condition generating abilities. With the spell Fragility, you can cause damage when they have conditions on them (poison, fire, bleeding, disease, blindness, etc.) Necromancers can create poison (usually self inflicted) and disease (elite skill). The necromancer can transfer conditions from self to others (self inflicted poison, then transfer it to others). Fevered Dreams can also spread new conditions on to other foes (elite). The rangers can use fire, poison, bleeding (elite) and inflict it onto enemies. You can also become a great trapper with a mesmer.
You can only have 1 elite at a time (unless you have an elite and capture other elites in the same area), so choose what type of character you want carefully.
Elementalists are good for creating spamable conditions (blinding flash), but sucks when it comes to self-healing.
Remember, you are a spellcaster, so your armor won't be very good. Stay away from melee if you can. This is why I wouldn't suggest an assassin, unless you can get in and out very well.
Hope this helps.
