There are few npcs around the behemoth(staff, h2h, great katana, the goblin). My theory is that you have to skillchain with them, not with other pcs. When you complete a objective the behemoth become vulnerable and it can be damaged for a 10% of his life.Complete 10 objectives and you become Heroic, after that kill the behemoth, talk to moogle and you'll get a Vanadiel clock and the kupofried trust(+10% exp and cp).