Gengar missing on Mewtwo Raid Guide

It is only viable (but also in fact, the best) against one moveset - Psycho cut and focus blast. In all other cases it gets one shotted by any charge move or dies with 3 confusions.

Much better going with a gyarados even if you're thinking of total damage output per Pokémon.

Bite Crunch Gyarados is there and is much better against focus blast variants.

Probably omitted because Gengar's performance varies so much. Its DPS is amazing but it usually gets killed too fast to be useful. Its best use is against PC/HB and PC/FB, where I would absolutely use it as it can often tank both of those charge moves once and will deal extremely high amounts of damage. It's strictly better than Gyarados against PC/FB and in fact even a level 30 Gengar is strictly better than a level 40 Gyarados according to sims.

If you happen to have a Gengar with very good IVs and leveled up high, it can take 3 Confusions from Mewtwo and becomes a worthwhile attacker against C/HB and C/FB also. The fact that its DPS is so busted means that if you get off those 2 Shadow Balls, its TDO is good enough, especially with groups of 5+.