Ayoub P It's been a long time since I played this, but, as far as I remember, Poisoned Weapon+High Level Crafted Weapon+Tier 3 Thousand Cuts+Potion buffs+Stealth is more than enough to do absolutely massive damage to single targets.
Try Lightning Flask + any bow. Switch to Sera, use Lightning Flask, switch to any other character and watch her destroy everything. Watch this: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-obo5pT664HY.html
I wouldn't call it a bug. Lightning Flask slows everything down around you. If you are not controlling that character, the character speeds up proportionally.