problem with the last challenge

[Gniarf]

You can have 1 chasing skull with the skull chaser ability.
PLUS you gain skull when your fury level reaches about 18.
PLUS you can breed them: when you reach lvl18, lower your fury to 15 (for example), get it to 18 again, lower to 15,18,15,18,15.....

I did the Serious Uncle Sam challenge with 4-6 skulls.