Yeah, Nix is definitely the challenge character to beat the game with. Max Power upgrade combined with the +1 Power from relics is a must. Without the relic bonus, Nix runs probably fail 95% of the time.
Grampshell increases resource spawn by 30% (Chests and Relics are not affected). For relic-related support, I think an accessory that grants additional extraction attempts could be useful, maybe something like +2 tries when extracting from relic tiles. For encased relics, personally my method is to try the opposite end if the first ping is weak, then try to triangulate the location of the relic (max 3 tries in a triangular pattern), then from there you should be able to get a strong ping and use the rest of your attempts around that spot.
As for the turtles, I personally feel Griddle is the easiest overall due to his high stamina pool. If you can reach the Magma zone with most of your stamina intact, you can just equip Winterheart and dig straight to the bottom without too much trouble.
