Going after two character unlocks in the same run can make things a lot tougher. Fern especially needs a lot of focus on collecting restore cells, which can distract from other goals.
For unlocking Grampshell, I usually go with titanium tools, Snowboots (or the new boots if you have them), and a Winterheart. I also stock up on Mageflare Runes and try to use one every 7–8 digs down once I'm in the magma zone. And of course, always use a torch.
Good luck on unlocking the fedora and diamond pickaxe :)