@PhineasB I definitely found the Holy Freeze merc to greatly increase safety overall, and his survivability in particular.
@Pb_pal used Delirium on his merc to great success, so you might want to try that. That's what I did too recently, but it didn't work that well. Partially because it meant he had no leech (don't have the runes to make a CoH for him as well), so he would need a big rejuv about as many times as he needed one without the Delirium, partially because it often spread out the Infector or Vizier pack, which resulted in more time needed to kill the packs. I can definitely see it work against Nihl though.
As for myself, I made some more progress. Not so much, still 'only' at 3.015M xp, but the runs are going well. I switched maps, so I didn't have the L-shaped Vizier map anymore. I really needed some time to get used to it initially, resulting in longer running times. There's so much more enemies in that map layout than in the L-shaped one. But after a while I did get used to it, and it's definitely fun running like this, because it really keeps me on my toes. I also often get to kill 1 - 2 boss packs near the seal, as well as some trash mobs. Not sure if I still get any xp of those, but they usually get killed in almost the same effort it takes me to kill Vizier, so I'll definitely stick to this map layout.
I also did get a shrine nearby, but don't bother with it. I tried a few times, but always get cursed immediately, removing the xp shrine, so it doesn't seem worth it for me.
One thing I found strange was that now, sometimes the Infector pack spawns very close to the seal, resulting in fewer minions in the pack. It's strange, because I had this layout for Infector on my previous map as well (the one with the seals more or less across each other, not the one with the Infector seal at the end, after a double right turn), but can't recall this ever happened on that map. Doesn't happen enough to bother and reroll though.
O yes, and the good news (because complaining always helps) ...
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Found them during a short, 40 minute session, with only run in between them !!! I still would have preferred a Ber for that Infinity, but I'll gladly take this for now

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These put me at 10 Mal, 4 Ist, 8 Gul, 1 Vex, 4 Ohm, 1 Lo, 1 Sur, 1 Ber and 2 Cham, next to the dual HotO, the CoH and the CtA I created for this character, so I definitely start to get a nice collection of runes. I might make a Fortitude for my merc, just to see if it makes a noticeable difference with the Treachery he is currently using, although if I'd do that, I'd make a non-eth for now, since I also want Fortitude for the oskill Sept I'm doing (and which I should pick up again).