Oh god I think I’ve ballsed it up after spending about 2 weeks on 2-1 and 3-1 plus mining 1-1 for souls, levelling up to lvl 20 I decided I could take on the easy boss in 3-1. So went into human form, nobody available to summon and in I went, die, rinse and repeat a few times until finally have managed to turn the world black and now have a red demon mind flayer thing that I can’t get past to even get to the bloody boss. Had enough now to be honest. Can’t kill any bosses other than the first one.
They're tough and hit hard, so not being able to kill it at a low level isn't surprising. Don't be disheartened.
Abandon it for now, or if you're on Reddit, ask for help on /r/SummonSign. They'll be able to help you with the boss, or improve your World Tendency by boosting invasion kills.
Failing that, I would personally either try to tackle 2-1 or 4-1. If you can get past the first few enemies in 4-1, and do the skip (search YouTube) to reach the boss, it's actually a very short and easy level. The boss is simple - all you have to do is hug the fog gate, shoot him with magic or a bow, and avoid his tongue attacks. You know they're coming because the yellow bird on his head makes a loud noise. You can always replay the level properly at a later time to collect items, an NPC and to fully experience the game if you want to.
Once you've beaten 4-1, you then have access to a very good early game farming location and a vender that sells some very useful items.
Early investment, in terms of soul and item farming, makes it a much easier experience; I spent a considerable amount of time grinding and still got the platinum trophy in under 30 hours, so I'd definitely say, while dull, it's a worthwhile time investment.