How to level a mesmer:
1. Equip a sword (mainhand)
2. Use illusionary leap 2 times to port to your enemy (if everything works)
3. Use blurred frenzy, followed by f1/f2/... whatever is available
4. Collect loot
Look at the tools you're given and use them! Mesmers aren't AoE nukers, they are cleavers. Killing 2-4 enemies at once is quite easy but more than that becomes hard (not that it matters much since the AoE-nerf but still a staff ele can just pull everything and be fine).
Don't bank on making a phantasm build, they are too slow for efficiently killing minions after another (instead of AoEing them down).
You can add flavor to "burst" mesmer by equipping a staff or greatsword in you second set. The plan remains similar as before but instead of starting with sword 3, you either berserker + mind stab first or chaos storm + chaos armor.
Thieves work in a similar way, you either go full on burst mode (d/d, steal combo) or full AoE mode with the shortbow (they both work but I prefer the dagger version personally).