From what I recall titans were busybuilding the wall. So that's why they didn't participate. A warlock bet a Hunter he couldn't kill one, so he did. It wasn't fully grown, hence "young ahamkaras spine". Other than that I believe fully grown Ahamkaras had some sort of mental powers hence why only warlocks went after them. Lore wise they'd have a way to stop it. The young one the Hunter killed had not yet developed these abilities.