If you have two mobs and need to pull 1, root the one you don't want to pull, when the other runs to you, FD. The rooted mob will immediately forget you since he's still at his spawn point (if mob level < 35). Stand up and proceed with killing the mob that ran to you.
Another good way is to mez (Screaming Terror) the mob you don't want to kill. At the lower levels, it will blur him very often and you won't even need to FD to kill the mob who ran to you.
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