Stun Threshold:

It takes quite a bit of damage to reach the stun threshold, but once you’re able to hit her during these moments of vulnerability, you can build up the stun meter faster.
Keep in mind that once you stun her, the fight becomes a lot easier, as she will be temporarily immobilized and unable to attack, giving you valuable moments to deal additional damage.
Stun Synergies:

Weapon choice and gear: If you have gear or POE currency weapons that increase stun chance or stun duration, be sure to take advantage of those in the fight. Two-handed weapons or high physical damage builds are usually best for maximizing stun potential.
Flasks: Ensure your flasks are ready to give you any additional damage boosts or curses that may help in reducing the Queen’s stun threshold.
Additional Tips:
Dodge and Avoid Damage: While focusing on building the stun, don’t forget to dodge her other attacks, especially the explosive fungi and debuff circles. Staying alive and keeping up your damage output is just as important as landing the stun.
Maximize Damage During Vulnerable Windows: The Queen’s debuff circle phase is your golden opportunity. Make sure you’re using high-damage abilities like Area of Effect (AoE) skills or powerful single-target slams to apply as much damage as possible during these brief moments.
In essence, stunning the Queen of Filth boils down to timing. During her debuff casting phase, focus on dealing as much damage as possible to reach the stun threshold. With the right gear and careful positioning, you should be able to buy POE2 Currency stun her and get some breathing room for the remainder of the fight.