/castsequence adds your latency to your cast times, as it has to wait for a successful cast to flag back from the server. The simplest solution would be to write:

/castsequence Living Bomb,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,
/cast Frostfire Bolt

It should cast Living Bomb once every 12 seconds, and cast Frostfire bolts before it comes up again. (I think bomb goes off after 12 seconds. If I'm wrong, adjust the number of commas by 5 per second in which ever direction I missed it by.)

You could theoretically do even better if you want to get in to /click functionality, but this should address your initial DPS issues.