Krenaht:
Marred Giants are unable to evolve any further, so they stop evolving. If a Marred Giant is slayed, that does away with the risk of them joining forces with the giants.
That's why we're going to slay the Marred Giants. Of course, you can select how much of a workload you can manage. Interested?

Basic mission - Defeat 50 Marred GiantsIntermediate mission - Defeat 100 Marred Giants
"I'll take the basic mission." "I'll take the intermediate mission."