The thread has moved on so I'll just explain here, I guess.
Amoonguss takes about 1/3 damage from HP Ice from scarfed Thundy, iirc. Honko's calculator isn't working for me right now so I can't confirm this calculation but it was upon my memory that Amoongus could counter Thundy to some extent. It basically counters anything it can take a 2HKO from, because of Spore.
If my calcs are wrong and Amoonguss can not take HP Ice, then I apologize and I will edit my post.