I saw a lot of people saying this game is to hard, so I'm going to give some basic and simple tips that can help you out. This might contain spoilers for certain types of enemies, if you want to meet them and learn from experience.
This guide is very simple and poorly done, someone else will spit something out better anyway. I hope it helps out.
Dashing and Movement
Remember, you have no restrictions in movement, you can double jump at the end of a meat hook, you can double jump anywhere in a dash, there's no limits to your movement, don't handicap yourself not dashing through the air.
Now Dashing is by far the most important survival tool in the game, the faster it recharges the better. also it only recharges when you touch the ground, if you dash once it recharges even faster the game even lets out a blip noise which might drive you insane but this tells you that you're dash is ready for use. Remember that it ONLY recharges when you touch the floor.
Dashing is perfect for dodging hell knights but if you're 1v1'ing a hell knight, when he's at a distance he will always leap at you if you keep moving backwards he's going to leap where you're going so if you take small step back and stop, when he jumps he'll land where you were and if you move back as soon as he jumps you can dodge it by moving backwards if there isn't a wall in your way. But its better to just dash out of the way.
Once you get the super shotgun the meat hook can be used for more momentum, it can also pull you out of close to death situations. Just Remember it can only Hook onto live demons not walls, not objects, only demons. Its last master upgrade gives you a burning hook which makes them drop armour great for latching onto an imp and getting some armour back.
Mistakes That Get You Wiped
Now we all make mistakes in combat even I do.
Now a big mistake I've seen, I did as well NEVER try to chainsaw an enemy that's above the amount of fuel you've got this leaves you open and you fall into a false sense of security thinking that its going to put you into the breathing space you were going for. But if you are quick enough you can dash out of it.
Always check your fuel down in the right corner but honestly I never chainsaw bigger enemies cause fuel is always very limited to pick up, so just chainsaw a zombie or an Imp, No you can't chainsaw Prowlers or Carcass's with one fuel. If you don't know these two enemies yet, you'll meet them soon enough.
Never ever back yourself into a corner, stay on the move the game wants you to keep moving.
I repeat DO NOT go near a Mancubus, even if you want to blood punch an armoured one they somehow activate a bloody AOE effect instantly even if you hit them with the blood punch so stun them in some way then strike.
Also never use the rocket launcher when a Carcass is around you're just signing a death warrant.
Those blue shields can Fluff you up.
Try not to use up the second dash it can get you badly injured or killed, dashing once is faster and better then using both. Only if you are dashing to an empty spot for it to recharge when you land.
Red glowing Buffed enemies, If you try to fight them under this effect you're going to die. You can still kill them its just 10X bloody harder.
What causes this is a totem or a Archvile, the Totem is a long red thing with a red energy aura spinning around it on the floor somewhere, I'll get a screen shot of it another time. The Archvile can buff all the demons in the area, kill him on sight
Another tip Don't engage the Archvile up close he just blasts you away, Unless you have the sword.
Do not Glory kill or chainsaw in the middle of a group of demons and don't do it next to a Mancubus as soon as you get out of the glory kill or chainsaw they do an instant AOE that can almost one shot you on nightmare. If you make these mistakes you'll end up like this.
Equipment, Runes and Equipment
I'm not going over board with this guide, perks and runes that can be a choice on how you want play.
Grenades and Ice Bomb:
I tend to do this sometimes I forget to use my Grenades and Ice bomb, but the Ice bomb was crafted by the holy VEGA himself, the Ice bomb freezes any enemy within its blast radius only one that it can't is the marauder and if my memory serves me right, the Doom hunter can only be frozen when his shield is down or you got him off his sled. Launching the Grenade into a group of frozen demons can cause a lot of damage to them all and if you're overwhelmed or cornered USE THAT ICE BOMB!
Now this is one of the main important features of the game, Chainsaw, Ice bomb, Flame belch and Glory killing. These three are you're Ammo, Health, Armor and Health. I know the game already explains these for you but it doesn't tell you that you can combine them.
Just a reminder the Ice bomb needs an upgrade so they drop health when you shoot them or kill them when they're frozen.
Now lets do some combos with the shoulder cannon and chainsaw.
Chainsaw and Flame Belch:
These two together will give you a ♥ ton of ammo but will give you like 20% of your armour but if you're hurt badly it doesn't give much health.
Ice Bomb, Flame Belch, grenade, Chainsaw. Now I would only use this if you're being overwhelmed by a lot of enemies.
When you freeze everything and if you've got the perk that makes them drop health when you shoot or kill them, then burn them they will drop armour and health, then if you fire a grenade down and then chainsaw one you'll get ammo, health and armour and when that grenade goes off its raining health.
But this is all can end up differently depending if you've got the perks that leaves them frozen longer or big enemies burn longer or if they unfreeze faster then normal, the bigger the enemy the quick they get out of it.
Just remember you can mix up these equipment items in anyway.
If you're hurt and got no armour with plenty of ammo, shoot down an ice bomb and burn them, shoot them or use a grenade.
I honestly prefer Air movement it allows me to dodge balls in the air without dashing and having Longer glory kill distance increases the reach for my chainsaw and glory kills, the last one I have my equipment recharge faster when enemies are under the damage effects or killed.
But the runes are up to you on what you want.
Read The codex After Your First Encounter with a New Enemy
I'm guilty to I was enjoying the combat to much that I didn't look at this and it could have made some fights much easier for me.