So out of Eriador, the High Elves or the Dwarves who do you go gunning for first and why? I've tried playing the OoG several times on VH/VH, but have never had much success with them - ordinarily I take out Imladris as this removes a major thorn from your side (otherwise I'd imagine the HE are going to keep harassing you at Coldfells and Gram, and it gives you access to high level buildings so you can finally get a diplomat). Ideally then I'd like to go after the Dwarves, as they have no one else to keep them occupied but you in the west (and they often like to start trouble from the east when Dain's Halls fall) but of course Eriador start sticking their nose in and then you have too many fronts to manage at once. Even if you did manage to take the Blue Mountains, I'd imagine you'd then have to deal with both the HE and Eriador simultaneously so I'm not sure this plan would be very sound.
As a side note how do you deal with Dain's Halls? For me it always falls to the Dwarves, and then they start harassing Gram, but my troops are always engaged in a westerly antagonism and so I find it pretty hard to protect Gram because of this (as most of the settlements are also on the western side of the Misty Mountains and so supply lines aren't great). I've tried protecting Dain's Halls with my own archers, and I've tried keeping a decent garrison in Gram - but both of these approaches are frustrating as they eat up money that you'd rather use taking settlements, and both fail after a while (the Dwarves keep sending more and more troops your way, and you can't replenish them at the same rate). Even if you took Dain's Halls from the Dwarves, I'd imagine then they'll just start harassing you for it every fifteen or so turns until you're ready to expand to the east and bring the fight to them, and so it wouldn't really be a solution to this problem. Ideally the OotMM would just hold their own, but I'd doubt they'd ever do that.
I guess how I normally go about strategizing is play all the different permutations until I find one that fits, still having more guidance from the off is always good as then you can slim down your approaches - so in other words if attacking one of these factions also really didn't work for you that would be good to know too. I think the optimal situation after taking Imladris would be to make peace with the Dwarves, focus on Eriador, then the High Elves if they're causing difficulty (in my experience they usually ask for a ceasefire when you no longer have borders with them) - and finally focus on the Dwarves at my leasure. Still, I've never found a good way of managing this and I would guess such an approach probably isn't possible (which is why I said I would actually focus on the western dwarves first above) so I'd like to hear what other offensive strategies others have adopted.