Without 20xx:
Practice multiple jumping bairs with fast fall between them (It will look faster and will make you place youself better for the following backairs) try using about 3 or 4 of your 6 jumps.
Practice jumping fast onto the platforms, and wavelanding back so you can drop from the platform with a bair or uair and fast fall (hbox does this sometimes, and usually catches ofguard the oponent).
Practice ALOT of wavedashing oos. so you can do it consistently.
Practice empty short back jumps into turn around grab or utilt as soon as you touch the floor (this is a good mixup if you do it after a couple of spaced bairs, specially if you see him shielding to much).
Practice shielddrop, shaidrop and just regular drop from platforms, and falling with an aerial.
With 20xx:
Practice smashdi foxes upthrow uair until you hands bleed, I mean it...
Practice wavedash oos against fox and falco shieldpresure (you can practice wavedash back or forward and then fsmash back or turnaround grab)
Set marth uptilting under a platform, and practice you shielddrop rest until you get it consistently.
And my favorite :3 spacies platform techchase into rest. you see that 20xx make the ai randomly tech and if its a spacie, it will shine on the first frame posible... well this is golden. It helped me improve alot my game.
Just upthrow fox or falco on the middle battlefield (after you master battlefield, you can try dreamland (harder since the platform are higher and youll have less time to reach and react, and yoshi is scary because if they techroll to the ledge, you may end up resting and falling of the stage). This is 100% doable, no need for insane reaction speed, and most of the spacies are crouchcanceling after they tech on a platform, meaning they will die on 0% when rested. My only advise is that you stay floating just below the place they would tech, and be prepared for the tech in place, if you see the tech in place just jump and rest asap, if not just move acordingly and rest. This works like a charm against shiek after dthrow on stages with low platforms like yoshis and FOD.
J
