Overview
In this guide, I will tell you where the best spot for grinding the level-, calypso-kill- and money-achievements is.
Grinding achievements
So you want to get
-Calypso to lv.50
-Calypso having 1000 kills
-the money achievements
-maybe some other killing achievements
The best spot to do this all simultaneously is at the very end of Inog Mako Daar. When you´re entering the Forbitten Chamber, turn around and go back to Inog Mako Daar. You will almost instantly run into a patrol of 3 Zamfir and 1 Star Priest. This group always gives you 1761 exp and 579 gold and most likely 1-2 Sanctum Swords and/or an Evil Rod.
The patrol is fairly easy to kill (at least in the easier gamemodes). To be most effective, equip Calypso with the Azgul Scimitar from the previous bossfight (=> counter-hits) and the Twin Stars from Quest 13 (=> 2 attacks). Then simply rush the enemies and when you´ve beaten them, enter the Forbitten Chamber, turn around and do it again.
You can also bring Om (also equipped with an counter-hit item), but be careful, as he is weak against their specials and mostly needs 2 hits to kill 1 enemy.
I did the Calypso achievements with Om at her side. For the remaining kills for Om (and also for the magic kills) I prefered to grind at the overworld.
You can also get the money achievements by buying the special content of the game. There will be a save file with plenty of gold. Start a game with this file and earn money by beating an enemy. This should be enough.