Spark Shock is an amazing punish. It comes out fast, has a huge hitbox, does great damage, and lasts long enough to trap a spotdodge. It's pretty much your go-to punish for most things. If you get the chance, utilt is your best kill move, if you can get it off of a punish, go for it. If you're too far away to Usmash but you still want to hit them, dtilt is a good gap closer that does more damage than lemons.
There's a thread on Crash Bomb and Leaf Shield setups on the main board, you should read them, it's a lot more in depth than I can go into here.
As for Metal Blades, there's a lot to talk about. First of, a blade in the hand is worth two in the foe. Literally. A Metal Blade tossed as an item will likely hit twice, while a normal tossed metal blade will only hit once. (I think this happens if you don't Smash throw it) If you have time, throw a blade downward and pick it up. If you crouch and c-stick forward while holding metal blades, you can start your pellet salvo, and once it's started, you can press A to continue it, move around, and jump. If you full hop a diagonal metal blade and follow it on its way down, it'll combo into a utilt for an easy kill.
More important than the blades are the lemons. Master these moves. You need to get to the point where you can basically move around however you want in the air while firing them. Also, practice short hopping at any point in the combo to continue it in the air.
Mega Man has trouble with a lot of the top tiers, although he's got some good gimp options against Diddy, you might have a hard time getting him off stage to do them. In addition to the big 3 of Diddy, Shiek, and Rosalina, Yoshi has super armor passively against small knockback, which means that at low percents, he can power through lemons and crash bomb explosions, and he has full super armor during his double jump which makes bair gimping effectively impossible. Surprisingly, though, Mega Man is great against Villager, who you'd think would counter Mega Man's style, but it turns out that you are a lot better at using your tools than he is, so as long as you don't give him an Fsmash, he can't use your stuff against you. Additionally, Crash Bomb explodes on contact with the tree, and Metal Blades go straight through it, meaning the tree is useless as a camping tool. Your lemons can cancel out his slingshots and gyroids. If it's a customs Villager with the ledge of death, you can take your Skull Barrier custom to reflect everything as you casually stroll over to him and stage spike him.
Good matchups for Mega Man are a lot of the almost-but-not-quite top tiers like Luigi and Ness. Luigi's shield slide means that a lot of your moves that weren't safe on shield now are, and Mega Man's a fairly heavy fighter, which makes Luigi's KO combos hard to pull off. Ness will have trouble getting in to grab you if you keep up the pressure, and remember to use your leaf shield.