I thought you said you got frustrated by dying in the middle? oh well.
The only way I could imagine to "warn" people would be to have the hill on a timer that corresponds with the timer of the fusion coil (e.g. when Hill gets to zero, mines explode). This, however, is impossible. Why? Well, because in Halo 4 you have a loadout camera. Unfortunately the fusion coil spawns before the game actually starts, but the hill timer won't start until players actually spawn.
I don't really see a need for warning players though. Basically, you just grab an overshield as quickly as possible, there's no real need to know exactly when that boom is going to happen, you just know that it's happening soon. Also, I increased the respawn time of the fusion coil a little bit so that gives players a couple more seconds to find a shield, unless they've all been grabbed already of course.
I played today with a bunch of randoms that I found in BTB and they said they had a blast. Let me know what you think. There's a lot less room for error now and I think it's more "obvious" as you suggested before. That first explosion now is also a couple seconds later than before which leads to fewer players getting mad by dying before they realized they had to grab a shield. Even if you do die in the first round, though, you will immediately know what to do in the second round and thereafter.
Just try it out again and let me know. If you don't like it, then hey, not everyone likes every game, but I'm happy with it and I hope other Halo players will enjoy it. Unless I find any serious issues, I'm done with this minigame and I'll be moving on to my next: "Clay Pigeons" on "Pull"