I just have the "1" on my hot bar. i throw a 1, shield lob, 2x flash and you should have agro. if someone is attacking anything but the one then you will just have to switch targets to keep agro, not much you can do about that. some people just tab target crap and make your life difficult. but just use flash after every couple rounds of your combination. as my target dies i just retarget my next target with a 1. most people are generally good at attacking the same target as you but sometimes you get a person who just doesnt pay attention.