Basically you get a bunch of fighters together (12 in this case) and get them to punch, kick, bite and thump the bejesus out of each other then you're close.
It's hard to say just why 2D fighting crossover games exist. Well, actually, I think I know why they exist (hint: it starts with a $ and ends with another $).
You are in control of a Predator who receives a distress call from Vega 4 of an attack on the city of New Shangi. Your goal is to wipe out all of the aliens before advancing to the next level.
In Strider 2, the player takes control of modern-day ninja named Strider Hiryu who wields a deadly plasma-ejecting cypher named the Falchion, completing missions assigned to him.