So I just unlocked the Sia continent and have hit my first real big wall. My parties power is 117,231 and I am not sure the best way to get it to 150K and beyond. Do I just save gems to roll for armor sets?
After taking a looksy at your kit you need more defense (aim for 65% defense) and attack (aim maybe for 30K+, if not crap tons more) in general, I'd probably max the 6 star sets you have to +10 then transcend them to where you can get them. Slot 5 transcended provides the most bang for the buck. Also I'd then roll for good mods. Minimum being orange, but you want pinks. Blue, green, and grey will no longer cut it.
Attack % > Crit Rate % > Damage % > Crit Damage %
Defense % > HP % > % Minus Damage Taken > % HP recovery
Ring/Brooch/Necklace (possible mod pulls change per slot)
Attack % > Crit Rate % > Defense % > %Damage to Boss > Increase Attack by X% of Y > Increase Crit Rate by X% of Y
Flat is fine if you can get them in pink, but percents are preferred. For weapons and armor you really want all those 4 mods, but I've prioritized them to what I think is most important, sort of.
After that, swap gear on your best characters. Thanatos' basic stuff, won't cut it for Sia. Some of that Serendi gear you have might be better on him, but you've got to get it to +10 with orange and pink mods as described above. So jelly that you've got Iron Fist of the East, but really weird to put both that and Outer Space on Chris, since they function sort of the same.
I'd get normal 1 star gear and get that to +9/+10 to fill out slots you don't have, but don't worry about modding them too much. Sell the 2 star stuff, and if you've got some 3 star sets focus on getting those and transcending them to +15 specifically in the Bow, Armor, Ring combo (it doesn't matter if they really match). If it's not a 4 star set or above, and you're not making a special build (i.e. Counter Build) then bow, armor, and ring is the generic way to go, translation: Crit build (aim for 100%)
If you must pull from the Summon shop, wait until I think May 17-19 which I think gives the chance to get Angel Knight and/or Coocoo Ranger, Go! That being said, RNG is a biatch in this game, and it may not be the best investment of crystals.
Climbing Tower of Dawn (ToD) for gear use to be good, but now there is a lot of 1 star drops that never existed. Still if you treat it like the daily summon pull, I guess it isn't bad, but definitely worse than before. ToD level 55 and 110 are the real meat now. 110 now especially since it's guaranteed to be either 5 or 6 star sets.
As a digression, I'd build up your Thanatos with Iron Fist of the East, Green Ranger, Doom Bringer, Black Ranger, Ultimate Gold Tiger. With that configuration he could solo Dimensional Breakthrough under the cover of Grace in the later stages. Ugh, also just saw my old post, Valkryie used to be better than Chris, but Awakened Chris is the new Tank Queen though Awakened Ildo does give her a run for her money.