Phantastic

I'm a big phan of this achievement.

On a serious note: you earn phantasma by killing enemies in Torghast at any layer. Every mob drops some, excluding mawrats and the last boss of level 6.
All phantasma earned, even if spent, counts towards the achievement.

This is quite a hefty sum of phantasma to get. Running both layers weekly will probably net you this achievement after 4-6 weeks. You can speed the process up by using the Anima Cell on the broker to get a solid phantasma boost, and by picking all possible phantasma offered while choosing powers.

Found a way to easy farm 2,5k per run. Go to the Fracture Chambers or the Upper Reaches and buy Ravenous Anima Cell from broker. After use in on mob to get Marrow Scooper buff. Kill sceletons and get tons of fanatisma! It's still gonna take some time to complete 250k, but it's much faster this way