1. You can roll or spotdodge out of the chain grab
2. his up b has a lot of landing lag, bair him when he returns to the stage (it sets up nice) if hes far below the stage, set yourself up at the apex of the up b (the top height) and bair him there. Never hit dedede while hes falling down or coming up, you will lose.
3. Weight is a problem. Camp the hell out of him to gain percentage. Nair is good up close (always land behind the shield or you will get grabbed) arrows are good far away. Watch for waddle dees, they eat arrows. Basically you're gonna have to have the percentage advantage. Save at least one kill move (not glide attack, killing dedede up is nearly impossible)
4. His reach isnt the main issue here, it's his priority and knock back. His bair has really good priority, it eats through arrows and is pretty spammable, it doesnt have knockback though. His uptilt comes out insanely fast and has knockback. Much like snakes... Watch out for uptilts out of shield. f-tilt and d-tilt, they have range, priority, but lack speed. These can be punished imo. The fair has slow speed and high knockback. A good dedede player will land this to kill you though. watch out when he approaches you in the air facing towards you.
5. Stage choice: You want to choose stages with either A) Floors that you can jump through (delfino, halberd) B) very little ground space (norfair, hanenbow) Or stages with high ceilings (jungle japes, hanenbow). Hanenbow fits all three in case no one else noticed. Japes is my 2nd fav stage to pick, norfair is my 3rd choice.