Unlock
Boss Battles open at level 50. The mode was expanded from 16 encounters to 32. Lionfish used to be the last fight. The writeup then names Craig Mamilton after Lionfish, then Granny Tentacles, Buff Sandy, Robot Mantis, Barnacle Man, Dungeon Master, Bubble Popping Boys, Nosferatu, and Plankton Tank Robot, ending at Sticky Fins. That paragraph does not name all 32. Do not assume the missing names.
Sticky Fins is the prismatic drop from the final encounter. It is not sold in the store and is not a standard banner unit. One earlier farming note listed a 0.05% prismatic Sticky Fins drop on a tier 5 boss clear. The unit page describes a line attack: 6 million damage every 0.1 seconds at base (60 million DPS) and 65 range, scaling to 15 million every 0.1 seconds (150 million DPS) and 97.5 range when fully upgraded. The chain pops and leaves ground spikes.
An endless community ranking still placed Sticky Fins with early and budget towers, below the top prismatic carries. Use the boss page for how to obtain it and the tier page for that endless grouping. They are different claims.
Why run them
Colosseum Clash achievement clears are the published chest pile: more than 100 Secret Chests, 21 Exotic Chests, and guaranteed Prismatic Chests, plus card packs, star-up potions, and raffle tickets. Boss fights also ignore the usual round-speed cap, so a strong roster can end them in seconds. Account XP from a normal boss clear is small, often 5 to 39.
Speed passes do not change boss timers the way they change story loops. Bring single-target damage for the boss and a slow or AoE if adds are the leak.