So why these combos? Isn't it better to just do the optimals?
These combos all follow a similar structure despite having very different effects, so they're easier to recall in an actual game. Once you get the muscle memory down all you have to think about are "what can I hit with?" and "what do I want to end with?". Furthermore, many of the true optimals (particularly those involving back-to-back edgestonish) have extremely strict and very character-specific inputs that it's not always worth trying to go for in pursuit of a relatively minor increase in damage. Some of the optimal combos are also the most practical, however.
What's with the numbers in parentheses?
Gengar's jY and RL X have multiple hits and can be cancelled into a Pokemon move (jY) or jump (RL X) after each hit. Whenever you see jY(1)YR or jY(3) or RL X(4), that's how many hits of that move you have to connect before you cancel it.
How does Gengar's combo theory go?
Gengar's most effective combos generally go something like this:
1. Launch - jAR, j2XXR, jY(1)YR, etc
2. jY - above, level, or rising
3. j6AR - single hit Astonish ("edgestonish")
4. RL X - 3 hits or 4 hits
5. jY - 1, 2, or 3 hits
6. Astonish - either single hit (j6AR) or 4 hit (j5AR/jYYR)
7. RL X (extra synergy) or RL A (attack debuffs) or 2XX (highest damage)
As much damage as you can fit in 21 hits (most of these combos should end in a phase shift with 20-21 hits).
If you don't have full meter you should generally end your combos with RL X to get extra meter.
If you have full meter already you should end your combos with Sludge Bomb (if you can do it) for the attack debuffs.
Going for max damage involves ending with 2XX, but the optimal versions of those combos tend to be incredibly difficult and you usually get more out of debuffs or extra synergy. However, I've included the practical max damage combos as an alternative when the others aren't feasible.
About first jY height:
The very first jY you do after you launch them has to be done from a different height depending on the size of the character. Thankfully this is not different for every single character, they all fall within one of three height tiers. All jYs after the first one are basically "as high as you can without dropping them".
Hit from above (-45 degree angle)
Lucario, Gardevoir, Blaziken, Weavile, Suicune, Machamp, Braixen, Croagunk, Scizor, Decidueye, Mewtwo*, Shadow Mewtwo*, Aegislash**
* 2XX ender does not work
** Falls out of RL X(4)
Hit level (0 degree angle)
Gengar, Sceptile, Empoleon, Pikachu, Pikachu Libre
Hit from below (rising jY) - cannot link RL X(4) -> jY in combos (no synergy ender)
Charizard, Garchomp, Blastoise, Darkrai, Chandelure
Duel
Midscreen
Astonish combos
jA
jAR, jY(3), j6AR, RL X(4), jY(3), j6AR, RL X(4) - 168 dmg + synergy (Recommended)
jAR, jY(3), j6AR, RL X(3), jY(3)YR, RL A - 162 dmg + debuffs (Recommended)
jAR, jY(3), j6AR, RL X(3), jY(3)YR, 2XX - 185 dmg
j2X
j2XXR, jY(3), j6AR, RL X(4), jY(3), j6AR, RL X(4) - 172 dmg + synergy (Recommended)
j2XXR, jY(3), j6AR, RL X(3), jY(2)YR, RL A - 167 dmg + debuffs (Recommended)
j2XXR, jY(3), j6AR, RL X(3), jY(3)YR, 2XX - 185 dmg
jY
jY(1)YR, jY(3), j6AR, RL X(4), jY(3), j6AR, RL X(4) - 178 dmg + synergy (Recommended)
jY(1)YR, jY(3), j6AR, RL X(3), jY(2)YR, RL A - 167 dmg + debuffs (Recommended)
jY(1)YR, jY(3), j6AR, RL X(3), jY(3)YR, 2XX - 195 dmg
6YX
6Y[X], RL X(3), jY(3), j6AR, RL X(4), jY(3), j6AR, RL X(4) - 193 dmg + synergy (Recommended)
6Y[X], RL X(3), jY(3), j6AR, RL X(3), jY(2)YR, RL A - 188 dmg + debuffs (Recommended)
6Y[X], RL X(3), jY(3), j6AR, RL X(4), jY(3), j6AR, 2XX - 199 dmg
RL X (does not work on Pikachu, Pikachu Libre, or Croagunk)
RL X(2), jY(1)YR, jY(3), j6AR, RL X(4), jY(3), j6AR, 2XX - 210 dmg (Recommended)
RL X(2), jY(1)YR, jY(3), j6AR, RL X(4), jY(3), j6AR, RL X(2) - 174 dmg + synergy
RL X(2), jY(1), j5AR, RL X(4), jY(3)YR, RL A - 172 dmg + debuffs
Meaty 5[A] (does not work on Pikachu, Pikachu Libre, or Croagunk)
5[A], RL X(2), jY(1)YR, RL X(3), jY(3), j6AR, RL X(4) - 186 dmg + synergy (Recommended)
5[A], RL X(2), jY(1), j5AR, RL X(3), jY(1)YR, RL A - 171 dmg + debuffs (Recommended)
8X
8X(1), jY(3), j6AR, RL X(3), jY(3), j6AR, RL X(3), jY(3), j6AR, 2XX - 262 dmg (Recommended)
6A
6A, Y+B - 123 dmg (Recommended)
6A, RL X(2), jY(1)YR, RL X(4), jY(3), j6AR, RL X(4) - 157 dmg + synergy (Recommended)
6A, RL X(2), jY(1), j5AR, RL X(3), jY(3)YR, RL A - 158 dmg + debuffs (Recommended)
6A, RL X(2), jY(1)YR, RL X(4), jY(3), j6AR, 2XX - 161 dmg
Non-Astonish combos
5X, 4A - 124 dmg
5[X], 2A - 180 dmg
5YY, 5A - 119 dmg
Wall
RL X, j8XXR, jY(3)YR, RL X(4) - 268 dmg + synergy (Recommended)
RL X, j8XXR, jY(3)YR, 2XX - 275 dmg
RL X, j8XXR, jY(3)YR, RL A - 230 dmg + debuffs
5X, 4A W!, 4[A], j8XXR, RL X(4) - 228 dmg + synergy (Recommended)
5X, 4A W!, 4[A], j8XXR, 2XX - 231 dmg
5X, 4A W!, 4[A], j8XXR, RL A - 196 dmg + debuffs
Wall (Burst)
5YY, 4A W!, 5[A], 2Y; 5[A] - 223 dmg + Shadow Ball oki
5[A], 5YY, 4A W!, 2Y; 5[A] - 242 dmg + Shadow Ball oki
Field combos
jfAR, RL X - 91 dmg (Recommended)
jfAR, bY - 77 dmg (Recommended)
jfAR, XX - 98 dmg
nY, 4[A] - 78 dmg
Field setups
nY; 4A
nY; 4[A]
nY; 5[A]
nY; sY