Fight hasn't changed. You can still use 3 time stops in a row and again after planar shift. Remember that you have 5 time stops you can use, just don't use them all at once. If you chain them correctly and your dps is high enough, Eregos won't have a chance to spawn any whelps. Of course there's a chance Eregos spawns whelps right after planar shift before you can get in range, but it's not necessarily game over even if you get 1 group of whelps. If you get Eregos to 2nd planar shift without whelps, you are pretty much guaranteed to get a kill.

It took me whole lot of tries before i killed him for the first time. Nowadays i usually fail only if server lag messes up my dps rotation.

Good luck, hope you get him soon!