Haha yeah it's quite challenging in the beginning. The best strategy is to buy 4 speed upgrades at the start then farm money / bonus rooms with a close to full light, then when you have something like 40 speed you can easily go past zombie/skeleton. Also don't forget to buy max health upgrades at the shop otherwise any monster will one shot you when reaching the darkest places.
Getting a "Drain" "Bite" "Throw" or "Heal" early also helps a lot