I successfully killed a giga using only Direwolves.
Level 50 giga, vs 37 direwolves, most were leveled to have 6k health and 500 melee. I could have waited for all of them to have it, but that would have put it off for at least another week, and I've waited long enough.
In the end, 21 of the pack did not survive, but the remaining 16 are about to get a ride home where they will rest, and then take on their new role as base guardians. The wolf who got the killing blow has been properly named as Giga Slayer, and she will live a pampered life inside my tower.