Frosslass isn't needed, but I chose her because it's blizzard is stronger than anything Gengar can pull out (a combination of 120 base on Blizzard vs 80 base on Shadow Ball and the fact that Focus Blast (also 120 base) doesn't have any STAB on Gengar)
However, Gengar has a stronger T-Bolt and Shadow Ball and Levitate. Mismagus, Dusknoir, and so forth should all be considered for your team.
But a ghost of one type is essential not only because of the fighting immunity, but because of _rapid spin_ immunity. A bolt/beam ghost like Frosslass hits the sweetspot IMO because of its ability to single-handedly demolish the most common spinners in the game. Donphan, Starmie, Claydoll... all are weak to BoltBeam. Hitmontop is an exception, and it can strike back with 80% accurate Stone Edge... which is then reduced by Snow Cloak.
I thought it might be wiser to use an abomasnow who uses leech seed + protect. As it's your lead you can protect scout your first turn. And you can switch out without just throwing away a subsitute (this is situational however.
I might be missing a huge advantage to the subleech set but as I see it now leech seed + protect might work better.
Ah, another pokemon who needs 5 moves...
Because of the way Walrein works, the last pokemon remaining on the opponent's team after stalling are usually Cresselia, Blissey, and other walls like that. That is why sub/seed works well on them. However, I also need a way to handle with cleric blissey, and I do that with Sub Focus-Punch. Focus Punch also works well with Freeze Hacks.
So it is a matter of Protect vs Blizzard vs Focus Punch. Which combination is optimal.