Hammer Bro is just so easy to counter, you have to get the timing down first. If someone's HAMMER HAMMER HAMMERing, just wait until he comes back down and STRIKE
I found a video of an Australian Waluigi player. He is SICK.
http://www.youtube.com/watch?v=Crll-G6IXgk
Most of these aren't really applicable in a real match, but Wall-Surfing is my new trick of choice and if that fails I go for the Wall-Magnified Shot. I might stay with the same trick I've been doing, which is a half-chip at the goalie, vine up to the ball and crack it in.
God, I love Waluigi. :=