In this boss fight, I'm playing as the Collector. How is that possible? By modifying save file and replacing my hero data with data of the Collector I've encountered before. Game data remain the same - boss stats are default.
Some details of boss mechanics:
1. The Collector can summon heads with 1-round cooldown. It's a forced skill, which mean when it's available, it will be the only option.
2. The chances to have the ability to summon depends on amount of heads already in the battle:
0 heads = 100%
1 head = 50%
2 heads = 15%
3. MAA ability to guard has 3-rounds cooldown. It's a forced skill and can target any position.