If I may make a few suggestions:

Try changing your main macro to look like this:

/stopmacro [channeling]
/click MultiBarBottomRightButton1
/click MultiBarBottomRightButton2
/click MultiBarBottomRightButton3
/click MultiBarBottomRightButton4
/stopmacro [exists,harm,nodead]
/assist yourtankname

This does two things. First, you don't have to have your channeling condition in each individual macro (personal preference on that). Second, your mage will only acquire a new target if he doesn't already have one. This allows you to switch targets on your tank and have your mage continue to attack his own target. Once the mage's target dies, he will look to your tank for a new target. You can supplement this with a simple /assist macro so that you can force your mage to hop to a new target in case he gets stuck targeting something you don't want him to attack.

Also, try consolidating your fireball and living bomb macros as follows:

/castsequence reset=target ,fireball
/cast living bomb

I was quite happy to discover that living bomb will NOT be cast on the target if the target already has living bomb applied. It will give you the "a more powerful spell is already active" error. Spamming this button will attempt to refresh living bomb every chance it gets.

One last change that you might try would be to consolidate scorch, fireball, and living bomb, all into one macro:

/castsequence reset=target ,scorch,,fireball,,fireball,,fireball,,fireball,,f ireball,,fireball,,fireball,,fireball,,fireball
/cast living bomb

This macro will reapply scorch every 30 seconds. It will attempt to reapply living bomb every time your global cooldown is ready. It is 100% spammable. I use a similar macro with my frostfire spec, and it works quite beautifully.