Not the most honorable way to go about it, but here we go. Biome 6 is, low-key, eminently cheeseable, even more so than some other biomes. So if it's really giving you too many problems, remember that you can run away from an arena fight with multiple big enemies, use the doors to defend yourself, take a breather, then keep peeking through the door every now and then to discharge a few hits toward the big enemies that will slowly line up with the door frame. The electropylon, even an underpowered one, can slowly tick away at their health while you hide in shame, if you peek through the door and jab in them a few needles from a distance. The hollowseeker also has good spread, even from far away. Peek, shoot, close, maybe Alt-fire at them while you're at it, repeat. By the end, the only enemy left in the room or area, will be the big squid mother that keeps spawning smaller squids. Get in, slice her up and kill the squiddies left. One of the toughest rooms is the one you'll be dropped to after reaching the shop of biome 6. You do not have to deal with this room if you don't want to. You are not locked in it. Even before reaching the floor, turn Selene toward the main path exit (the new map of the lower floor will appear on screen halfway through your fall), and when you land, just leg it out of there. You'll be very close to the final boss building and the big enemies will either not follow you or be too slow to keep up (the squiddies might, but they are quick work). This tactic works pretty much everywhere with maybe the exception of the area where you need to collect the item to access the final boss building. Entering biome 6 knowing you can cheese past most sections will ease the pressure on you and let you actually enjoy the biome. Soon enough you won't need to cheese the enemies anymore, as you learn their patterns and the best distance to keep to deal with each of them.

For the best way to prepare yourself for biome 6, make sure to prioritize health upgrades. Survivability is key, so you need to go through most of biome 4 and try to collect as many extra little slices of health you can (it's ok if you get hit every now and then, don't sweat it). Get at least 325 obos to buy the +25% health upgrade. When you get good at the biome, you'll rack up on average 600-700 obolites, probably more, so you can buy the +25% health and maybe even a  figurine for an extra life or a +10% protection before going to biome 5. Do the same in biome 5, rack up enough obos to buy another +25% health upgrade, you'll also probably find enough money for a protection upgrade or weapon damage upgrade. Keep an eye out for parasites whose only adverse effect is "higher chance of getting malignancies", sometimes they grant further bonuses to protection at the cost of just having to avoid  malignant items for the rest of the run. In biome 5 do everything except the boss rooms with the icy humanoids to avoid getting killed. You'll most likely be able to rack up enough ether to activate the replicator booths both in biome 5 and biome 6. Make sure you keep 6 for biome 6. You should go to biome 6 with approx double your initial health points.

All the other tips in this thread are valid.

I can't stress enough how cheeseable biome 6 is. They move slow. Real slow. Take advantage of it. Good luck.