Enemies on the Crystal Planes provide notoriously less XP than on other higher level regions, but the enemy density is quite bigger in comparison, so my guess is that it evens out eventually. Plus you have a lot of quests, and once you go deeper into the region you'll find some undead and some mini-Facetalleons that give better XP. The area nearing Narnuil's lair is a nice place for grinding XP. And it makes sense for a Pyro Elf to dominate on that region since it's mostly an icy one, but beware because a lot of enemies have physical/poison and physical/magic resistances and damage so not everything is icy. If you want a great place to grind in the Wastelands do the following; start at the harbour and head over until you reach the first monolith with the merchant near it. You'll have to cross over a bridge and you can see a great deal of demons below that bridge. Now the trick is this, cast Shadow Step and target the spot below the bridge, you'll see your HE is suddenly down there in the middle of them. There are a lot of enemies there so beware, but once you've dealt with them you can travel back and forth that path and you'll basically do a perpendicular path to the one you do over the bridges while doing the path of the main quest. That path below the bridge runs for a long way so you can grind a lot of XP there. I think you can easily do 1 million there, so that's that. When you get over that just teleport back to the monolith and keep going your way.
Answering your question to Hooyaah, at first it's important to master the skills to have the mastery bonus kick in, then if you really need them to be of a higher level you can do it. But the point is that there's a big jump when you go from 74 to 75 hard points on a skill, the same not being exactly so from 75 to 76 as the payback is considerably smaller. So I think that first and foremost it's important to reach mastery on the essential skills of the build and afterwards start pumping those two or three specific ones that will always come in handy (extra points in Constitution/Toughness for added HP/armor at levels 150+ for instance). The only reason to no not stop at mastery level at first is that you absolutely need to keep the skill at character level, either because you're a bargainer/blacksmith/etc or because you need those modification points for a fourth or fifth CA right away. Other than that it's 75 first and then we'll see.