I think ive figured why´. I tried beating him with 17 chiefs (each 45 damage) and 16 beer guys (204% boost) took him down to 8k hp. However I noticed the boost in damaga was aditive not multiplicative i.e 204% boost was dealing 133 damage not 90ish damage. Then I tried another island and the damage was 95. which is actually correct.
*This only applies when multiplier is above 100% and is a rough estimate
TLDR: Normal = damage*boost%
Boss = damage + damage*boost%
 "Frozen Islands"){kind=icon}