All information in this guide is courtesy of /u/vbcnxm_ on Reddit
You have to trigger all 7 status effects (Burn, Confusion, Poison, Rage, Stun, Weakness, and Wound). If any enemy or player has all 7 at once, it instantly dies.
First things first, you need to redecorate. There is a certain game room item that reduces the number of status effects needed to cause Sudden Death down to 6: The Weapon Rack. You should buy this immediately!
Next, you will need to build the character. The specifications are as follows:
Ninja class
Human race (preferred)
Jock player (preferred) (Any player works though.)
1 skill rank in Black Arts (a passive Wound 1 on crits)
1 skill rank in Shadow Chain (a 3-in-1 attack)
21 skill rank in Vanish (bonus to crit chance)
You will need the following items:
An Axe (Weakness and Crit +2%)
A Hammer (Stun)
the Viper Tooth (Poison 2 on crit) (found by searching the Meadows)
the Volcanic Rock (Burn 2 on crit) (found by searching the Steam Flats)
the Troll Snot (Rage on crit) (found by searching the Nameless Bridge)
the Pocketwatch (Confusion added to attacks, lowers threat by 10, raise crit chance by 2%.)