This is a really good team for a mono-ghost so here's a few things you could consider changing:
Banette
Banette has this incredible ability once it goes mega known as prankster, (hopefully you are thinking trololol with this thing) It has +1 Priority confuse ray at it's disposal so why not use it? I would recomend replaceing protect with this. You could also use will-o-wisp instead and turn banette into a staller
Trevenant
Nice moveset for a good stall, beware of prankster taunters because they will ruin this.
Aegislash
This is a pure sweeper and will get you alot of wins if used properly, once again beware taunt
Froslass
No. Just no. That will not work, period. so try this:
Froslass (F) @ Focus Sash
Ability: Cursed Body
EVs: 4 HP /252 SpA / 252 Spe
Timid Nature
-Blizzard
-Shadow Ball
-Destiny bond
-Pain Split
Cofagrigus
Will serve you well as a Trick Room setter and will sweep until Trick Room runs out
Gengar
I can see the strategy you plan to use this for many times and it probably won't work unless you can set it up against something it will work against such as Kangaskhan. And don't you already have enough pokemon carrying Will-O-Wisp? It is a good move I agree but this is a bit much... Also this Gengar has incredible sweeping potential so why not use it? I recomend this:
Gengar (M) @ Life orb
Ability: Levitate
EVs: 252 spA / 252 spe
Tmid Nature
- Hypnosis
- Dream Eater
- Shadow ball
- Sludge wave
Overall I would say good team though there is no weakness coverage beware of Dark. To counter this might I sugest using a physical set of froslass with wake up slap to be used after gengars Hypnosis? And also beware taunt. Alot of your team has a stall strategy and that could be your defeat
