What is required to open boss chests?

Winning the fight is only half of it. The chest appears once the boss is down, but the loot inside stays locked until you spend energy on it, and that spend covers both the rewards and the drop materials. If you are out, the run gave you the fight and nothing else you can carry home. Plan the energy before you queue, not after the boss dies.

The same applies to ordinary chests in Holo Battle challenge mode, so a long session in there is metered by energy rather than by how many runs you can win. Getting into the fights is not the bottleneck. Boss challenge events support matchmaking for teams of up to four, and you can go in as a duo, so there is usually a group available. What limits you is how many chests you can afford to open once you are done.

If you are chasing Glimmer specifically, boss battles, chests and the Holo Battle Simulator are only some of its sources. Quests, daily tasks, events, elite training, puzzles, branch activities and the battle pass all pay it out too, and daily tasks and events are the efficient way to farm it. That makes energy-gated boss chests a poor choice when Glimmer is what you want. Save them for the drop materials you cannot get elsewhere.

Not settled: The resource is labelled both Primegy energy and Prime Energy in the closed beta build, and the cost of a single chest is not stated anywhere in the mode.