I dont ever notice the ''weakness'' in my mesmer. I mean, im literally fighting about 3-5 mobs at a time, uncluding a veteran or two in there. I find it no stress or hassle to take down a large group of mobs.
If anyone remembers in Lornar's pass that group of like 12 Grawls on the ice in a circle just chillin, well run in there with a staff and Greatsword and the jobs done. You need to find out how to roll the mesmer, be it more illusions or just more dodging. Idk, I picked up on it instantly, its a lovely class. Main for sure.