Crabs are tough - I’m not a top tier player, but I’m not aware of a killer strategy like for some of the mobs. I do find that a small amount of upgrades makes a big difference in the early game, though. I’ll usually choose the item from the ghost that I can use the soonest (lowest tier) unless I’ve already found an equivalent.

Don’t be afraid to spend an upgrade or two early - many players will advise you not to “waste” upgrades on early equipment, but most games you don’t need every upgrade on your end game equipment to win. If playing as mage, dump an upgrade into your staff right away; you’ll retain that upgrade, even if you imbue a higher upgraded staff.

Likewise, don’t hoard health potions, seeds, etc. - they’re no use to you if you’re dead. All items are useful - if you’re not using an item (seed, stone, etc.) , figure out how you can add to your gameplay.

For dwarven halls (after 3rd boss), have a way of dealing with ranged enemies; either a way to quickly close the gap (e.g., hourglass, chains) or a way to blind them (e.g., darts, wand of prismatic light).

Good luck!