First, as far as advanced techs go, what would be a general order to learn them in as far as effectiveness AND simplicity?
Second, reverse dolphin slash is when your opponent is in front of Marth, and you hit Up+B and the opposite direction at the same time so you hit the opponent in front of you but launch backwards. Why is this a good thing? Is it more powerful, have less lag, an alternative to a bair ledgeguard (jumping off the ledge and hitting your opponent with a bair), or just simply give you more space between you and your opponent if you miss?
It's not any more powerful than the dolphin slash, but the dolphin slash has a sweet spot, and it's easier to hit when you reverse it. Stand something like 1 to 2 feet away from your opponent when you do the reverse dolphin slash and you should sweet spot it. It's excellent for KOing space animals and it comes out very quickly so they rarely see it coming. It's not so useful for edgeguarding--stick with ledge-hopped BAIRs, neutral B, fsmash, and dtilt.
As for the techs--
1) Short-hopping is essential for Marth's aerial game and shouldn't take too long to master.
2) Dash-dancing is easy to learn and fairly useful for Marth, moreso when combined with other techs like wavedashing, foxtrotting, and pivoting.
3) Jump-cancel all your grabs. This is very important for Marth since his grab game is huge, and it's not difficult once you get the timing down.
4) L-cancel all your aerials. This is relatively difficult but not once you get used to the timing.
5) Wavedashing with Marth is not that difficult and is very useful for spacing and mindgames.
6) SHFFLing is less useful for Marth than other characters but should still be learned. As a tech it's slightly harder than wavedashing.
7) Ledge-hopping BAIRs is important for Marth's edgeguarding game, and ledge-hopping NAIRs or FAIRs important for returning to the stage aggressively. Ledge-hopping NAIRs is probably the most difficult.
8) Short-hop double FAIR is one of Marth's more difficult techs but is extremely useful for chaining FAIRs across a stage.
9) FAIR to DAIR--not sure if this is a "tech" but it is a very important combo for Marth. Chain FAIRs into DAIRs for low % kills. It's hard not to kill yourself when you start doing these. In general if you double-jump immediately before using the DAIR you'll be able to come back to the stage.
10) Pivoting is very difficult and useful only if you can do it consistently. But if you can do it consistently it is very useful, as you can pop out fsmashes while dash-dancing.
I'm sure I'm forgetting something but there you go.
