HAI GAIS
Ok, first thing to realize is that G&W's smashes are not punishment kill moves. i.e., he cannot punish anything of your moveset with any of his smashes, but rather, G&W lands his smash attacks through reading your mistakes. In other words, he guesses, based on your habits and patterns. So when playing G&W, once you pass 70%, pay attention to every move you make and camp hard; if he's approaching, he cannot hit you with a smash attack unless you are an idiot.
He MIGHT be able to punish a powershielded fsmash with a smash attack of his own, but you really shouldn't be throwing out fsmashes like that.
You can drop shield and jab (utilt if facing away) before the grounded hitbox of his bair comes out if you drop shield when the turtle's head dips. You can Smash DI out of his bair if he hits you while airborn and punish it with a down air. Your fair is also really good for punishing bair; remember that bair goes inactive when the turtle's head dips, so if you simply pull away and then fair once the head dips, it goes right through.
Use little aura spheres at medium range; if he buckets them, you get a free fsmash or whatever you want, and you force him to approach.
Learn to tech dthrow every time.
And never, ever roll toward him. Rolling away is actually almost always good because of his bad run speed. No projectile + bad run speed = GL punishing a fast roll away. However, you don't want to corner yourself by rolling to the wall, because suddenly you have to make a choice to get past him, which gives him a read opportunity to smash you.
To beat G&W, you want to recognize what situations are "read opportunities"; situations where a guessing game occurs. G&W always wins read opportunities. I mean, he might not win the guessing game, but he is rewarded three or four times more for winning the guessing game, so it's not worth the risk of engaging in a guessing game. Learn to recognize guessing game situations, and then focus your game on preventing them from occurring. If you remove the guessing games and reduce the match to spacing, then your character wins. Don't get backed into a corner, and think out every move you make. Remember, G&W has to approach; he has no projectile; as long as you are not approaching or trying to get past him, he has to approach you and you can react. He has no solid approach, and bair sucks on shield, and nair is only safe on retreat.