I think the above solution works but I haven't actually tried firing a charge up attack using that method. Instead I just held the fire button for a weapon and it charges up automatically. I charges up 3 bars before the charge dissipates. If you let go after the first bar you get the first attack, 2 bars the scond attack and 3 bars the third attack. Just fire 100 of these charged up shots, either 1, 2 or 3 bars is ok, and the achievement will unlock.
This method is much easier than the above button combinations.