Don't use mantle in a solo setup. You need the heal from Coils of the first spider while batting AND the damage reduction from Lakumba's Soul Harvests.
Yes Arachyr has been buffed. That's why you'll see mostly arach players on top of the leaderboards.
Gems: Try for yourself what feels best I'd suggest. Right now I'm running Trapped/Stricken/Taeguk because I clear trash fast enough without Pain Enhancer. If I get to a point where this becomes an issue I'll switch but right now it's good. Get yourself a 25 PE (don't need a higher level here) and try all combinations. Use whatever you like most.