best advice,space your self,then fake him out when he uses the turtle,rolling behind him,or backing off and stutter smashing him work well,G&W lives next to the edge and he'll do anything to get you there so just keep away from edge ta all costs
he'll almost always down throw>D-smash so roll out the way immediatly,and his F-smash can still hurt you after it has already comeout for a good second
fireballs works but not 2 at a time cuz that will lead to the bucket,and eventually that bucket neing full
fireball aproaches are good still tho,scince his bucket has some small lag after absorbing an item,just make sure you've already landed in his face before he puts the bucket away or you'll get a hammer in your face
just keep calm and dont compare %'s he'll die faster than you do
G&W is anoying especially his up-b,if your to proud to counter pick(like most mario mainers XD) keep practicing against him
hope this will help...