Argument in Lelouch's favor courtesy of GANON1025:
Lelouch has an answer for everything Anti-Spiral can do.
1. Sending out Mugann/fighting himself
Lelouch is invincible, as in nothing Anti-Spiral or his things or whatever can harm him. Any attack that hits him turns into nothing, in a manner of speaking, but basically yeah he's invincible. And, even if he WAS hurt, even KILLED, Lelouch can revive himself instantly. There's the inherit immortality and regeneration from being the Black King, then there's The Dead Raise geass power that he can use on his own body even after he dies.
As for attacking, Lelouch has his own geass The Zero. By touching something, Lelouch can destroy it infintely. The only thing shown to resist The Zero had special geass protection and similar immortality to Lelouch which Anti-Spiral won't have. His flight plus geass powers like The Speed means he can keep up with the mugann. The Zero not only destroys the physical, but the metaphysical and interpretive. For example, when he gains his new powers Lelouch is contracted to abide by and serve under Eden Vital (gateway to heaven/alt universes, basically an almighty power). But using The Zero, Lelouch renders the agreement into nothing so he gets off free. AS may be able to go to an alternate dimension and send mugann through it (which is a dumb argument that undermines MPFC at its core and basically should not be allowed), but Lelouch can destroy all of them and he will not tire or become fatigued so he can continue to do so until AS appears himself. If he doesn't, Lelouch can use Eden Vital to cross into different dimensions. He can look into at and see different dimensions at once, so he can find which one AS is in.
Additionaly, Lelouch can read the past, present, and future of Anti-Spiral so that will probably come in handy. Lelouch can also use one of Nunally's powers to create a copy of himself for other shenanigans as well. AS is very arrogant, in direct combat he will try to match Lelouch's power instead of going overboard with any kind of super super robot thing. Even if he did ride a huge robot, Lelouch is still invincible and it just makes a bigger target to use The Zero on.
2. Anti-Spiral sends Lelouch into alternate universe / charade
Lelouch is a wired, and as such he has a special connection to a place called Eden Vital. Eden Vital is a sort of gateway through which characters, specifically Nunally and her geass, can look at and interact with alternate universes and other dimensions (such as Heaven). Nunally's geass is one of powers Lelouch is confirmed gaining by becoming the Black King. It would easy for him to break through Anti-Spiral's charade. This is an intuitive connection, even if Lelouch doesn't immediately realize what's wrong his wired connection would alert him to the shift in universes. This should also kick in for illusions and such, if he believes in it enough to be real his connection to Eden Vital will work as if it is real too.
--
Currently playing: League of Legends, Chrono Cross.
Current Projects: Fool's Overture, CYOA: http://www.gamefaqs.com/boards/8-gamefaqs-contests/60143513