What happens if my opponent has Malamar with the "Contrary" Ability active and I play Will?
Will lets you choose the first flip for an attack as Heads, but then Malamar's Contrary will treat it as Tails.
Source: TPCi Rules Team (2022-03-03)

Relevant cards

Note: All applicable cards may not be listed below.
XYP_58
XYP_58
SM12_208
SM12_208