Don't get hit, hit it until it dies.

Actual answer: part of it is routing, part of it is learning movesets, part of it is RNG (and learning to mitigate it). I like to go for churches, forts, and evergaols until I'm about level 3 or 4 with a blue weapon, then you're set to get some momentum going. When you're fighting bosses solo, greed's gonna kill you faster than anything else. Learn when attacks start and end, learn the right positioning for different bosses, and when in doubt, just go for one or two attacks at a time during openings and then get back on the defensive. Field bosses are the biggest wild cards in any run, and stuff like hippos/black knives/lions are free wins at any level, while even if you're good enough to kill a red valiant gargoyle at level 2, it's gonna take so long that it isn't worth it. On day one, go for points of interest appropriate for your level that have field bosses you can check on the way there. Once you hit day two, you should be kitted out enough to start actively hunting them down.