I would say, try to blend your highlights a bit more, I personally can't stand seeing such obvious paintbrush marks on a miniature.
This of course doesn't apply if we're talking tabletop quality, I think your Archaon probably looks awesome when it's on the table.
When you're talking
OSL, I guess you want to talk about the hot lava glow on the Horse right ?
If that's the case, two things:
1. It shouldn't end up on the top of the horse's head, the lava is below.
2. It should be a lot fainter, horses aren't exactly mirror like, even when sweating. It's a daemonic horse so he may even find lava to be just fine in terms of temperature