I've done this before wth friends on another story so i'll see if this works.
So, this isn't a fanfiction. Basically you become a mean guy of choice (titans, giants, whatever) or a nice guy of choice (CHB campers, Percy, eg) and you set up an army and fight the other person.
It'll look something like this.
TEAM 1: titans, giants, etc
TEAM 2: campers, gods
Kronos: "I freeze time and I send Hyperion to hold back Percy Jackson"
Percy: "I feel like I move in slow motion and I see Hyperion coming, but luckily Thalia saves me by shooting an arrow in his eye"
Rules:
No changing sides
No rebirth/regeneration
Can have appropriate swords/shields (eg. Thalia, aegis)
You can't say something like "I am uneffected by Krono's final blow." BUT, if you have dipped in the river Styx (remember to get the blessing, and tell at least one person your mortal spot) then that's fine.

So it's just the Titan and Giant war all over again, but it's combined. I'll see if this works, If not then I'll delete it.

You try and kill the other team, and once one team has no more remaining people then the game is over.