hmm... then 15 seconds is fine... if you are cornered by 2 tyrants, and they slash you about 4 times, you got over 800 damage on you... this means, when invincibility wears off, YOU ARE DEAD NO MATTER WHAT...
So >.> if thats the case, 15 seconds is good for suicidal missions AKA killing OM!
Even better idea: Invincibility Pack
Cost: 400-500
Time until wears off: 20? seconds (can leave it 15)
Damage Taken After 20? seconds (can leave it at 15): Whatever you took, for example, take 50 damage, once the effect wears off, you have 50 HP
Second Effect: Grenade type Explosion of the body (400 damage, all around? Maybe 300-350)
Health Boost from Invincibility Pack: 50