I just thought I'd post some tips that worked well for me, maybe they will help some people.
If you're hitting a "wall" where you don't feel like you can live long enough to get enough gold for an upgrade, I recommend you just ignore enemies and look for chests. They show on your map if you're in the same room, so when you enter a room just check the map to see if there's a chest. If there isn't, move on to a different room. If you have spelunker unlocked you can see all the chests on the map from anywhere, so you can easily avoid rooms with no chests.
Boss door rooms are great since they have two chests, which can contain armor blueprints or decent chunks of gold. Plus there are no enemies in these rooms so you don't even have to fight your way to the chests.
Early on enemies aren't worth killing for gold, they drop like 10-30 gold if you're lucky which just isn't very much. If you can't take an enemy out without getting hit, then it probably isn't worth fighting.
Always use at least one mobility rune. You need these for a lot of the fairy chests, which are important to get. My favorite is flight, because you have full mobility in any direction. Also I find it nearly impossible to dodge some attacks without any mobility runes, so I really can't recommend you fuse them enough. I don't recommend mixing extra jumps with flight, but any other combinations will be good.
Vampire runes are better to use than blood armor, especially early game when your HP won't be upgraded much. Eventually you'll find a blueprint for
which is good enough to use in place of/along with the equivalent runes.
The Shinobi/Hokage is a great early game class. With the bonus to every physical attack you should be able to one shot most of the enemies in the first area or two, and once you get enough points into attack you'll be able to destroy almost all the monsters in NG.
If you're dying too easily, I recommend barbarian king/queen since they have so much HP that you can afford to take a few hits here and there.
Smash all the chairs/tables/vases/etc. you find. The gold coins will add up, and you can even find bags on money on occasion. You can also find chicken drumsticks even in random destructible objects, and they can mean the difference between dying in the next room or making it to a couple more chests.
There is a clown you can find who offers two different games that you can play for a prize (armor blueprints). The axe game seems to be basically impossible, and the dagger game will take a lot of practice, and even then is quite hard.
Spellsword exploit has been patched, the axe game really seems impossible now
If you have Spellsword/thief unlocked, you can completely clown on the clown's games. This is because when you are given the weapon for the game (axe/dagger) it becomes your main spell temporarily and using the spellsword's special ability you can throw unlimited axes/daggers, and they'll be giant which makes the game even easier. I say you can throw an unlimited number because for some reason if you use the regular spell during the clown game it will refill your MP bar.
There are sometimes secret paths through walls that will lead to a chest. You can tell if there is one nearby if your map shows the room you're in to be much larger than what you can actually access. The paths are entered through small breaks in the floor that you can jump into/fall through.
Dwarfism is a great genetic for getting chests/fairy chests. If you look closely you will sometimes see small secret paths that only a dwarf can fit through, these paths either lead to dwarf only chests/fairy chests, or shortcuts that let you bypass most of a room.
Be warned that the Dwarfs have appropriately smaller attack range, which can make fighting some enemies tough. On the flip side, they also have a smaller hitbox which can make dodging some attacks much easier.
As far as which skills to invest in, I don't really think it matters too much since they all make the game easier. If you really want max efficiency I recommend putting your points primarily into attack until it gets too expensive, then crit chance, then whatever stats you feel like you need most. Being able to oneshot the early game enemies makes things much easier, since you can hopefully kill them before they launch their attacks.
Investing in the skill that gives you 10% more gold per skill point is actually quite worth it, the extra gold adds up quickly. The one that lets you keep 6% of your current gold per skill point when entering the castle is not at all worth it in my opinion, unless you've already bought everything else.
This might be obvious to others but you can go back to the skill point allocation screen even after you're in the blacksmith/rune lady area, you just walk all the way left. I didn't personally figure this out for a long time, and I always worried about my money management on the skills screen as a result.
There is an unlockable class at the very top of the skill screen which I personally recommend you do not unlock, because it is terrible in most situations and will litter your character choices. The class sucks because it doesn't have a regular attack or regular spells, if you're wondering.
If you're on NG+2 or higher and having a hard time, use grace runes. Personally I would equip 4/5 (if you have them) and use armor with skill points to replace vampirism/mobility runes.
If you have a hard time with the "bullet hell" type attacks, Barbarian King/Queen's shout will clear the screen of all projectiles which should let you get some hits in on the offending enemy. This also works for the boss of The Maya.
That's all I can think of for now, hopefully some of that is helpful to somebody.