I also was not happy with the big assist button in FTL so I modified my macros this way as well. To fix your macro try:

Code:
/Castsequence reset=6/target [nomod:ralt,nomod:rshift,mod:rctrl, target=TOONA-target][nomod:ralt,mod:rshift,nomod:rctrl, target=TOONB-target][mod:ralt,nomod:rshift,nomod:rctrl, target=TOONC-target][nomod:ralt,mod:rshift,mod:rctrl, target=TOOND-target][mod:ralt,mod:rshift,nomod:rctrl, target=TOONE-target] []Lava Burst,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
(change the FTL modifiers to the ones you use)