I read someplace you need to put a comma at the END of /castrandom lists:

i.e.

/castrandom Lightning Bolt,Flame Shock,

Also spelling and capitilization must be exact.

This should prioritize Lava Burst:

/castrandom Lava Burst,Lava Burst,Lava Burst,Lava Burst,Lava Burst,Lava Burst,Flame Shock,

If lava burst is not up it will cast flame shock, if it is up you have a high probablity (6:1) of Lava Burst instead of Flame Shock. And you can make that ratio as high as you want with macaroon I would think