Good question!
I think the answer is to look at where and how the North has defeated the Ironborn on land:
It was too late for that now, however. Theon had no choice but to lead Asha to Ned Stark’s solar. There, before the ashes of a dead fire, he blurted, “Dagmer’s lost the fight at Torrhen’s Square—”
“The old castellan broke his shield wall, yes,” Asha said calmly. “What did you expect? This Ser Rodrik knows the land intimately, as the Cleftjaw does not, and many of the northmen were mounted. The ironborn lack the discipline to stand a charge of armored horse. Dagmer lives, be grateful for that much. He’s leading the survivors back toward the Stony Shore.”The sea was closer, only five leagues north, but Asha could not see it. Too many hills stood in the way. And trees, so many trees. The wolfswood, the northmen named the forest. Most nights you could hear the wolves, calling to each other through the dark. An ocean of leaves. Would it were an ocean of water.
Deepwood might be closer to the sea than Winterfell, but it was still too far for her taste.If it were me, I would take the strand and put our longships to the torch before attacking Deepwood…
“My queen,” said Tristifer, “here we have the walls, but if we reach the sea and find that the wolves have taken our ships or driven them away …”
“… we die,“Something flew from the brush to land with a soft thump in their midst, bumping and bouncing. It was round and dark and wet, with long hair that whipped about it as it rolled. When it came to rest amongst the roots of an oak, Grimtongue said, "Rolfe the Dwarf’s not so tall as he once was.” Half her men were on their feet by then, reaching for shields and spears and axes. They lit no torches either, Asha had time enough to think, and they know these woods better than we ever could. Then the trees erupted all around them, and the northmen poured in howling. Wolves, she thought, they howl like bloody wolves. The war cry of the north. Her ironborn screamed back at them, and the fight began.
And we had other help, unexpected but most welcome, from a daughter of Bear Island. Alysane Mormont, whose men name her the She-Bear, hid fighters inside a gaggle of fishing sloops and took the ironmen unawares where they lay off the strand. Greyjoy’s longships are burned or taken, her crews slain or surrendered…
The Ryswells and the Dustins had surprised the ironmen on the Fever River and put their longships to the torch….
Harren died at Moat Cailin. One of the bog devils shot him with a poisoned arrow…
In Moat Cailin he had taken to wearing mail day and night. Sore shoulders and an aching back were easier to bear than bloody bowels. The poisoned arrows of the bog devils need only scratch a man, and a few hours later he would be squirting and screaming as his life ran down his legs in gouts of red and brown.
Behind him were the camps, crowded with Dreadfort men and those the Ryswells had brought from the Rills, with the Barrowton host between them. South of Moat Cailin, another army was coming up the causeway, an army of Boltons and Freys marching beneath the banners of the Dreadfort. East of the road lay a bleak and barren shore and a cold salt sea, to the west the swamps and bogs of the Neck, infested with serpents, lizard lions, and bog devils with their poisoned arrows.
While the Ironborn are stronger at sea, the North is stronger on land, and the problem for the Ironborn is that the North is nothing but land. The Ironborn don’t have the numbers to occupy the North, and their soldiers don’t have the training or equipment needed to fight the greenlander way.
If it I was giving strategic advice, I would tell the Northmen to surrender the coasts after carrying off everything edible and burning the rest, retreat into the interior, let the Ironborn spread themselves thin by trying to occupy the North.
Then once the Ironborn are over-extended and as far from the sea as can be arranged, ambush their patrols and attack any force on the march, besiege every castle and starve them out, burn their ships and cut them off from the sea.

