Definitely instances. I was able to hit up my first 5 man last night (Utgarde Keep) and boy was it difficult. Having done the same instances, even heroics a lot and fully understanding every pull and every boss I was shocked when I would die I never did get the last boss as he could AOE one shot all my guys (aoe did ~9500 - 10000 damage) when he went to undead form. Likewise I think the Nexus last boss will be quite difficult, _though_ stacking healing stream (vs mixing healing and mana) may work well.

I'll probably try out Nexus and see how that goes and if it goes well I'll try UK when I hit lvl 71.