Just use a targetlasttarget macro with the desired tank's name and it's pretty easy.
Code:/stopcasting /tar {tank's name} /cast Leap of Faith /y I choose you, %n! /targetlasttarget
That's the macro I use (obviously the yell isn't required but I like pretending I'm playing Pokemon). You can also get Marked for Death right before the Smash so being close to the boss isn't always an option; mostly you just want to avoid your tank taking a fire bath and dying. But using this macro you should only really lose a single global and you can go back to DPS quickly (just keep an eye of for the cast, it's a pretty long cast and he does an animation so it's pretty easy to spot).