An Avalanche attack is when you have enough people in your active party and are lined up in the battle gauge (on the left) one behind the other. Applying the "Weak" weapon types in your combo to specific enemies will decrease the Guard Break gauge on the enemies (visible after Harley analyzes them). This might be easier to do when you're a lower level, but even in higher levels, depending on the resistance some enemies might have, you can find that the weakness brings the Guard gauge down much faster. I'd wait to get this before overleveling into the triple digits.