I Wouldn't Want to Fight me Neither. Everytime Flash dies, he gains a skill point based on what killed him, or resistance to the damage dealt to him. Killed by a strength based melee weapon? Flash gains a point in Strength. Killed by a bardic spell? Flash gains a point in Charisma. 

Adaptive. If Flash has a 20 in a stat, he instead gains resistance to the damage dealt that kills him. Burnt to death? Comes back resistant to fire damage. Etc. 

Unkillable. Flash doesn't stay down for long. When Flash dies, he true ressurects 1d6 weeks later, fully clothed and healed. He also gains a permanent 1d20 to his maximum health.

Hard Hitter. When Flash has resurrected 5 times, his fists deal 1d6. 8 times, deal 1d8. 10 times, 1d20. 11 times, 2d20. 12+ his fists deal an additional 1d20 of whatever type of damage killed him last time. (Dissolved in acid? His fists deal an additional 1d20 of acid damage.) 

Consequences of Your Actions: If Flash has gotten to the point of being resistant to all damage, and a 20 in all stats, start giving him immunity to the damage that kills him. Immune to everything? He isn't magic. He can't teleport. But he is always walking towards the party.

Dedicated. Flash does not need to eat, drink, or sleep. 

Multiattack. Flash makes two Fist attacks.

Fist. Melee Weapon Attack. Reach 5ft. Target 1 creature. +0 (strength based) to hit. 1 (+Strength modifier) bludgeoning damage.