The story before you enter the Mantid-Bossroom:
Dalles gives you a taste of the demonic power to defeat the boss.

Now How to get the "Fact Up Achievement"
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1. Kill the mantid completely, means his 2nd phase too! (Important, to trigger the achievement!)
2. Sequence
3. Now
- use ur boost until its completely wasted (means let it go to 0)
- WHILE its charging -> die!

PS: If ur boost isnt full after the sequence, just run around till its full-loaded.
Use it then till 0, WHILE charging back again -> die!

For all the achievement-hunters here, i hope i could help you out with this and congrats you have the
special-hardcore-3%-achievement now 🙌