I just finished the slave mission in Denerim, I've killed the High Dragon,  and I think I'm about to go to the Landmeet. I'm a dwarf warrior at level 17 right at the halfway point of level 18. Should I go to the landmeet or go back and do some side quests and grind to get to 20?

I did a pretty good amount of the side quests and hit a little over 20 easily. If you did none of the side quests, you might not get there. Go do some and you should be golden.

I vote you play through it normally as you are, then get more levels during the expansion. 
Unless you're not feeling a burn then in which case you might aswell go for it. And that expansion is raising the cap so you won't be stuck there forever.

I'm not feeling a burn. I'm more than happy to go back and do some side quests.

I'm not feeling a burn. I'm more than happy to go back and do some side quests.

Ah I vote go for sidequests, never initially thought of the fact you're going for the achievement. 
Though if you don't make it, expansion will be out later this year.

I beat the game at 22. I did a bit of extra leveling to get through the last part of the game, but you'll get 2-3 levels from the last area for sure if you fight every enemy.

NEVER

I was level 21 and I did most of the side quests on my first playthrough, so chances are decently high you'll hit 20. So yes, go and grind!