LoL if you say you can’t get past it with SP, you just didn’t try hard enough.
I’m solo-ing dungeons everyday in my hard difficulty SP and got some goods out of them, like free 50-100 black pearls per run or free premium items (2-3 per run). And fyi, i’ve just started playing this SP for more than 2 weeks.
Just build your base on top of the volcano, farm metal like crazy, get yourself at least 2 set of good stats flak, a good sword/pike (both is best), a good crossbow/compound bow, and a raptoring good pump shotgun (for medium-hard difficultly i meant). Then there come foods, 3x canteen for the long dungeons without water, 1 stack of cooked meat is enough. The rest you need will be enduro stews, focal chilies and if you really need them, medical brews. Ah, and manage yout weight wisely
Keep in mind that the time limit won’t run out that fast, so take your time, learn the dungeons, try to kite and kill the threats with as less dmg as possible.