Proper DI- When your hit horizontal, mash the c-stick down and jump. When vertical, mash the c-stick down and fastfall. If your hit by a move your pretty sure will kill you, immediately airdodge then use the bike in the opposite direction of the way your going to halt your momentum(only works for horizontal)
Combos- There are many things you can do here. Full hopped dair to uair. Sh uair to up-b, dair to dair, bite to DACUS, nair to f-smash. Be creative >_< (you can watch some combo videos for ideas).
Waft setups- on characters with poor air mobility, try waiting for them to airdodge then while their vulnerable fart. At 0%, SH up-air to full waft is guaranteed on some characters for an easy 59%. Other then that you just gotta predict or use it when the opponent is trying to recover.
Glide tossing- Wario's glide toss is decent. I think its best if you throw the item RIGHT after you input the roll(almost simultaneously). You can glide toss forward, down, and back into a guaranteed f-smash(if the tire hits and stuns them) or maybe into a waft(havent tried this much)
Wario's best stages IMO are frigate, brinstar, Rainbow cruise and smashville. Worst stages IMO are luigis mansion and yoshis island.
I think wario's hardest matchups are ddd, marth and game and watch
Many people have grab releases on wario(even wario can get a free waft on himself but that goes against the wario honor code). Some of the ones you should watch out for are bowser(usmash,fsmash), ddd(fair, usmash), peach(usmash), snake(nair,utilt), ike(utilt), marth(fair,nair,fsmash), and falcon(free knee FTL). Those are the nasty ones that come to mind. Then some chars like yoshi and zss can infinite, but I've never actually had it happen to me so yeah...