This is actually fairly easy economically on H/H.

Ally with the Welsh. Take Dublin ASAP and make it your capital. Take Derry, then finish kicking the English out of Ireland. Make peace with England. Wipe out the Irish next.

Don't try to hold any territory in England or Scotland, but whenever you see an undefended city, take it from the sea, sack it, sell all the buildings, set the tax rate to very high, and move out.

Build up all the islands and all the cities in Ireland so that they have good sea trade. Since the AI is no good at invading by sea, you can get away with a small military. Build your treasury to about 100K.

Now invade Cornwall and sweep across southern England while the English are fighting the barons and the Welsh. These are cities you can hold.

At this point you are #1 in every category. The Scots have only cities that have been depopulated by your raids. The Welsh are your loyal allies. You cannot affors to garrison your cities against a full stack, but you can afford an invading army that can finally take and hold Scotland, even though the Scots have full stacks of troops all over the place.

As soon as you take region #31 (or is it 30) the Welsh backstab you and you probably lose. But it's been fun.