Do you think I could, using all the Greyjoy (and any neutral) discard top of deck cards out there, get an opponents deck to 0 before they got 15 power?

Here is a deck I put together quickly:

Character (34)
Bloodthirsty Crew (OSaS) x3
Desperate Looters (SA) x3
Drowned Disciple (KotS) x3
Drowned Fanatic (WotN) x3
Euron Crow's Eye (TGM) x1
Harbor Thug (TTotH) x3
Iron Fleet Raiders (MotA) x3
Seaside Urchin (TTotH) x3
The Knight (TftH) x1
Theon Greyjoy (WLL) x1
Wharf Rats (CoS) x3
Motley Crewman (IG) x3
Godswood Attendant (KotS) x3
Satin (WLL) x1

Location (18)
Corpse Lake (TBC) x3
Training Vessel (TIoR) x3
Frostfang Peaks (TWH) x3
Bloody Keep (KotS) x3
Iron Island Fiefdoms (KotS) x3
Sunset Sea (Core) x3

Event (8)
Called to Arms (CbtC) x3
Nightmares in the North (RotO) x2
Ours for the Taking (IG) x3

You can… but that doesn't mean you'll win. I've seen it happen where someone discarded the opponent's entire deck before their opponent got to 15, but they were not able to get to 15 themselves before they opponent was able to use what was left over to win.

I could be wrong, but glancing over your list I think your gold curve is to high and you may find yourself not being able to play what you need.

Milling is about limiting your opponents' options, but it does little to actually control what is on the board, or what they have in hand. There are a few choice cards that can help with this. With the discard theme, you really need to find room for Alannys Greyjoy (ODG) for some cancel. Maester Murenmure is another good one to include for some cancel during Marshalling. Either version of Asha is good with this deck, but the tri-con could be better if we include a few attachments also. But before we get to that, some other characters that can help you win challenges would be Wex Pyke and The Sparr. As for attachments, I would definitely recommend Support of Saltcliffe. Getting to stand Asha, Wex or the Sparr can be great for winning challenges or protecting the power you do have.

You could also go a slightly different direction if you have the cards available and use the Maesters path. The Brass Link is deck discarding on demand and with other chains like Copper Link, you can use Horn of Dragons to take your opponent’s character and use them against them. Speaking of, the HoD is a +4 STR attachment and works wonders on The Spar or Wex, more so with Support of Saltcliffe on them.