I didn't find the achievement that much of a grind.  The hardest part was just not being enticed into getting achievements in other games.

Good job you are a perfectionist you inspire me

Nice, even I didn't have the patience to get that achievement...and I've S-ranked 90 games...

I recently achieved 'Perfectionist' in Just Cause 2.   A dualism of emotion came over me when it popped.  The one side was happy that the most time consuming achievement for the game was finally under my belt.  The other side was sad because it knows that without another achievement to get, this game will fall to the side as others step up to be consumed.   
 
Just Cause 2, is a great deal of fun.  After the story is complete, and you are set in the free roaming Mercenary Mode, you spend all you time hopping around the island blowing up anything that is related to the established government.  It's very relaxing to spend a few hours launching rockets into fuel tanks, and you can't help but smile when sitting in a Auto-cannon armed Razorback when a fleet of foot soldiers descends upon you. 
 
Now only a couple of difficulty related achievements stand in my way of an S-rank, before Just Cause 2 becomes another: "Ya, I played it".