Destiny 2 Season of the Wish | Into the Light
So theres a fair few cheeses which can make Legend Onslaught a lot easier.. Including a safespot for the boss encounters, and quite a few ways to manipulate the batteries including duping them for IDU health and points. And even using them to heal buildable objects, or enemy combatants (though idk why you would do this?).

Battery Dupe Methods:
1- Use any class ability as you throw the battery into the IDU. This works the best with radiant dance machines.
2- Pick up a secondary battery as you throw the first. This will respawn the first battery allowing you to go infinite until the timer expires.

Boss Safe Spots:
1- On the right side of the boss arena. Jump on the horse statues then onto the ledge above. This out of bounds area has some great spots to shoot at the boss from safety.
2- At the back of the arena behind the same horse statues. You can stay in this area without enemies aggressing onto you.

Battery Interactions:
1- Batteries can be thrown at turrets or decoys to heal them. A direct hit is needed for this to happen.
2- Batteries can be thrown at champions or boss targets to heal them. Don't do this.

Tormentors:
1- Tormentors will get easily stuck on decoys and their melee damage is very weak due to npc vs npc scaling so the decoy will hold them easily while you clear the wave.
2- Tormentors move very slowly before their shoulder crit spots are broken. Use this to your advantage to clear the rest of the wave first.

Intro: 0:00
Battery Dupe Methods: 0:11
Boss Safe Spots: 0:49
Heal Buildables: 1:23
Tormentor Handling: 1:48