The orb for this fight is for enemies to take damage from a buff/debuff that ticks down. Some prominent examples of this are Celes' LD, Ace LD, Leon LD, & Porom LD. Exdeath's LD and S2 do not tick the orb, however the BT effect does.
This fight is designed around Ace, and it shows given how his LD works.
There has been a lot of chatter in the community over how complicated Ace's LD is / how it works. To put it simply, once you press the LD, let the boss have one turn. This will cause his LD trap to trigger, and replace itself with a new debuff that will tick every turn, for six turns.
His LD also grants him a special HP+, this HP+ reapplies the first debuff/trap from his LD. So once the six turns of his LD debuff expire, you can press his HP+ and get another set of his LD debuff, effectively giving him 6 uses of his LD.
It might take some time to try it out and understand in practice, but don't try to overthink it, it is much easier to use in practice compared to reading about it.
This team comp quite effectively dealt with both the orb mechanic, and the boss mechanics, as they will periodically put up a shield that needs to be destroyed before they get another turn. Between Exdeaths Gravity and Ace's LD trap triggering + Celes Ice imperil / enchant, the shields get destroyed rather quickly.
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