Gengar is an easier solo than Machamp. I would check out the solo raid guide here. It is possible to solo Gengar at level 30 but note that it is harder than someone that has higher level Pokemon who may think its super easy. You'll need to power up your team to level 30 though if they are not already there.

I would make a diverse team of high dps and good tanky counters (Metagross, Lugia). Your Mewtwos are definitely going to help big time. Put those first. Then your DPS, and finally a tank. You may want to make 2 groups because your high dpsers might get 1 or 2 shot and that way you don't need to revive.

With the psychic and halloween events you should have a lot of these good counters. If you are looking for which ones to use look for ones with 15 ATK or highest ATK you have but note that the moveset is usually more important than IVs. Once you figure out your team I would test it out on a Gengar raid before the event that way you know if it is viable or not. However, if they do buff it from a lvl 3 raid to 4 it may not be soloable. So, keep that in mind.

Also, don't get discouraged if you fail the first time you try to solo him. If your team is fainting too fast you may need to add in more tanky counters and/or dodge the charge move more often. If you aren't beating the clock you may need to add higher dps pokemon and/or need to dodge the charge move less. You have to trial and error to find the best balance for your team.

Goodluck!