Help with a Timed Style Combat with Fail Condition
Ok, so my first DH campaign is coming to it's final act (Tier 4). I've been planning out the details and scenarios for the last few adventures. In one of them I want the PCs to have to fend off a wave battle until a certain amount of time has passed, but critically I want them defending a McGuffin from these waves who will occasionally try to and perhaps successfully attack it.
So my initial thoughts obviously landed on countdowns. One counting up for the timer, one counting down for the loss condition. The loss condition is pretty simple. Each time a successful attack hits the McGuffin it ticks down. Set it at a reasonable number compared against the time one and maybe offer the PCs a chance to heal/repair the McGuffin as an action if they want as a kind of safety net. Cool.
The problem stems from the timer countdown. Every countdown I've used so far has either been a progress or consequence countdown or tied to an Adversary, so it's nice and obvious when it should tick. But what constitutes a "round" in DH, or time in a combat sense? Should I just use how many times the spotlight is used (ie how many actions are taken during the combat)? But that just feels like I have no firm grasp on what is a reasonable number of actions for the event, how many actions on average does a combat take? I could tie it to each time I take the spotlight, but that feels weird because each time things get "bad" they make progress on their goal. Do I just make it a Progress countdown? That feels weird there isn't really anyway for them to help the McGuffin do it's thing faster. Do I just count a round as some kind of vague number of players + maybe half the adversaries and the thing ticks every time the spotlight has moved that number of times? That feels more manageable than just straight spotlight shifts, but it becomes weirder to keep track of, as I'd be using a dual layered countdown (one tracking the number of "turns" and one for the actual time) and that just doesn't feel good.
So yea, I'm curious if anyone has done something like this before? How did it go for you? What tips/ideas do you have? Even if you haven't done something like this before how would you?
For additional context, the world is a setting where demi-gods called Childeren (spelling intentional) walk the world and offer strange Pacts with people who wish to practice magic. Each Childe is an aspect of the god they come from, which are the planets. All PCs that use magic have a Pact. One has a Pact with an entity known as the Lady of Ephemeral Fancy, who was believed to be a Lunachilde, but the player wanted them to be weird and mysterious and communicate only through dreams and visions. Earth is the only planet in our solar system that is not a god.
Through gameplay, their ideas, and my own I settled on the idea that earth was just not a god yet. Part of the plot has been the evil gods (comets, black holes, the empty void) trying to corrupt Gaia and stop her from being born. This also gave me the idea to turn the Lady of Ephemeral Fancy into a Childe of Gaia. Now she's finally being born in an astral mirror realm, a kind of idealized earth. But the evil gods are trying to stop that and sending minions to do so. The PCs are going to arrive right before Lady Eph is born, a weird flower pod containing her, but it's under assault by the enemy and I want them to hold out for a bit of time while a demi-god is born and prevent them from getting corrupted or killed.