You can't
There is no Mt. Sliver on FireRed
Firered don't have a "solar cave"
you dont use it it only unlocks a cave
You can get Mewtwo in the Cerulean Cave.
altering cave,
in the land cave.
Cerulean Cave
bartonvilleType your answer here...
mewtwo is in mystery cave :D
The only dark type Pokemon you can get in firered is murkrow which happens to be in lost cave on five island.
icefall cave
icefall cave firered only