Get Cemetary Arena:
Finish Level 2 with a Slayer Rank

Get Initiative Arena:
Finish Level 8 with a Slayer Rank

Get Quarry Arena:
Finish Level 11 with a Slayer Rank

Unlock Characters:
Get the following characters by completing the missions indicated with the rating indicated:

Mission Rating Character Unlocked
1 Professional Male Vampire
1 Slayer Female Vampire
2 Professional Zombie Skeleton
3 Professional Zombie Demon
3 Slayer Tara
4 Professional Bat Beast
4 Slayer Zombie Devil
5 Professional Materani
6 Professional Psycho Patient
6 Slayer Sid the Dummy
7 Professional S&M Slave
7 Slayer S&M Mistress
8 Professional Faith
9 Professional Zombie Soldier
9 Slayer Kakistos
10 Professional Abominator
10 Slayer Chainz
11 Professional Zombie Gorilla
12 Professional Joss Whedon
12 Slayer Chris

Get More Stuff:
Finish these missions to get the item indicated:

Mission # to Finish Reward
1 Anthony Stewart Head Interview
2 Amber Benson Interview
3 Nicholas Brendan Interview
4 Robin Sachs Interview
5 Chaos Bleeds Comic Book
6 James Marsters Voice Over Session
7 Anthony Stewart Head Voice Over Session
8 Amber Benson Voice Over Session
9 Nicholas Brendon Voice Over Session
10 Robin Sachs Voice Over Session
11 Joss Whedon Voice Over Session
12 Out-Takes