I find it hard to see this as even. It's 55:45 Snake IMO.
Snake can outcamp Pikachu and force an approach, which already puts him at a disadvantage. You can just jab tjolts or powershield if you prefer. Eventually Pikachu will get frustrated from getting hit by nades and just approach. Pikachu's approach is limited. He can shield approach and try to grab you. He can fair approach. He may use sh autocancelled dairs, or he could just try to dash grab you. That's pretty much all he's got here. Snake's pivot grab isn't even hard to land if Pikachu tries to grab. Pikachu's range is so limited. He may try to FF fair you to whatever combo, but you can either shield or hold a grenade and he will fail hard. QACing is just like asking for a jab or utilt to the face. If Pikachu starts jab comboing you, you get a free uptilt on him. However, a smart Pika will only jab you once so you trip. You shouldn't have trouble killing Pika; a few ftilts and grab techchasing and Pikachu will be ready for an utilt death.
The biggest problems Snake has here are in the air or when he's being chaingrabbed. Pikachu is a terrific juggler with uairs, nairs, and even bairs. These are usually led in by a dthrow, utilt, or a FF fair. Seriously, learn to not get hit by FF fair. It leads into utilt, dsmash, grab, ftilt, and even dtilt. If he is juggling you, he can also dsmash before you hit the ground to catch you in it and give you less time to DI out.
And then he has thunder... the way Snake recovers can make this tricky, but it's definitely avoidable if you stop your recovery just before you reach it and fall directly downward to the ledge. Avoid being uptilted at high percents because you will probably be in too much hitstun to get away from being thundered off the top. The key for DIing away from thunder is to get as low and horizontally far away as possible. A lot of people focus too much on airdodging and end up getting killed off the top. If you do get too low, the Pikachu will most likely decide not to thunder because missing could mean eating a bair as Snake comes down. He could still try to hit you with the base of thunder, which has a lot of kill potential, but you can see it coming if you just exercise good judgement.
Sorry if I am a little scatterbrained. Hope that helps.