Time stop does indeed have diminishing, as it'll be way too easy to lock out Eregos for 50 out of 60 seconds if it didn't.
However, it is still possible to use 5 amber drakes and do a "kill him before he kills you" thing. I would recommend saving 2-3 Time stops for whelps (I believe that's how frequently whelps spawn, I can be wrong though), and the other 2 being popped on cooldown despite diminishing returns.
Time stop's secondary effect adds 5 counters to all targets, which will allow the player who did time stop to effectively one shot every whelp. This is pretty important for clearing up the whelps, as they'll add a lot of additional damage, and without Rift or counter stacking amber's damage is terrible.