Catalyst allows poison decks to output much needed burst damage, at the cost of being slightly unwieldy. Practically a necessity for pure poison decks to beat the 3rd boss.
- Catalyst possesses a synergy with Nightmare, allowing much greater scaling of poison damage.
- Catalyst possesses synergies with Well-Laid Plans, Runic Pyramid, since it reserves Catalyst in hand to be used at optimal turn.
Act 3 bosses have counter-mechanisms for poison damage. The Awakened One / Time eater dispell every debuff/poison at phase 2. Donu and Deca are not a single entity.
So tactical usage of Catalyst is needed. Especially when it's Time Eater, Recommendation is using Catalyst at a turn which is phase 1 and you can kill it in 2 turns.