Back when I was playing, I would assist off of the tank and use Shaman Malo and Enchanter single target Mez to lock down a mob. You used to be able to tie it to a single key with a delay for the mez, but I am not sure if /pause works any more.

So basically, you are pulling with the tank and you select the target to mez with him/her (I pulled with my SHD, so usually I would pull a group with one of the Darkness line of spells that was a snare/DoT meaning that the target with aggro came to the cup last). Then I would hit the key for the SHM to /cast Malo (I know the syntax isn't right, bear with me) and also for the ENC to /pause 4, /cast Mez. So hitting one key cast the magic debuff and mez. Usually I would also try to get aggro with the tank via some DD spell.

It is important to adjust the /pause so that the mez lands just after the malo. Fizzling spells would obviously mess up the macro but then you just re-hit the macro.