Hoyo should really optimize how they list skills descriptions and boss mechanics on the skill description page. It’s a mess of long skill names self referenced in the description that bloats the text box when it’s fairly simple in how it works.

>Form 1
Summons adds. Has a counter that goes to 4 that increases whenever memosprites are added to the field or additional enemies appear (designated by the icon in the action bar) at 4 stacks Lygus switches forms.

>Form 2
Constantly summons adds. Counter is extended to 10. New action icon appears. Whenever new icon takes action ultimate’s power is strengthened. When 10 adds are killed ultimate activates and damage is evenly split among all possible ally targets. Switches back to form 1.

Lygus takes increased damage each time form switch occurs. Each form has an attack that action advances a target(s) and fills their ult to assist with clearing adds.
I may have missed something but I hope this helps people who got lost reading nonsensical names in the skills and not understanding the fight.
Clear was fine, big Herta never got the ultimate fill so I think it could be faster.