Well, since it involves waiting for the right moment, you could have a timer loop that accepts input, and you'd code the damage output depending on the time passed. It'd look something like this...
'THE VARIABLE 'T' REPRESENTS TIME PASSED IN FRAMES
FOR T=0 TO 120 'WAIT 2 SECONDS AT MOST
IF BUTTON() AND #A THEN BREAK 'EXIT WAIT LOOP WHEN USER PRESSES A
WAIT 1 'WAIT UNTIL NEXT FRAME
NEXT
0そうだね プレイ済み