Hi. This is a work-in-progress guide, and maybe even not actually a proper guide, but rather just a list of some useful information about The Haunted, that I've found, been using, and decided to save on Steam for myself, to not lose and forget, and use in the future, but ultimately decided to share with the rest of the Steam users, because someone might find it handy and use it too, so here it is.
Also, please note, that English is neither my native language, nor is it even a second official language in my country, so my English is quite far from perfect, but I hope you'll understand and will be able to use the information I provide in this guide.
Alright, let's get to what you've come here for already )

- If you are (just like me) having hard times dodging the Abbaddon's fire attacks in the final fight with him, after you beat 4 rounds ("Encounters") in Inferno game mode, you can actually go straight to the boss fighting stage to practice it! To do this, simply open the Console by pressing the "~" key in the Main Menu of the game (or in any other menu or right in the middle of the fight - it doesn't matter when and where you do it, actually), then type "open TH-BossLevel" (without the quotes, and it's also not case-sensitive, so you can bother not with the capitals if you don't care - I wrote it with them just for the better readability here) and press Enter. This will get you straight to the boss fight arena, where you can practice dodging, skull-kicking, etc. as much as you want! (Sadly, you won't have any weapon upgrades, so it won't be too easy to defeat him.)

- If you press the "Z" key on your keyboard during the game, your character will immediately die - this key is bound to the action "suicide" by default. To prevent sudden and unwanted deaths by accidentally pressing that key, you can unbind that action from it (and leave it unused, bind some other action to it and/or, probably, bind the "suicide" command to some other key, in case if you might need it in some situation (when you get stuck or want to quickly free yourself from the monster's body in Demonizer and other game modes that allow you to play as a demon).
To do that, go to

C:\Users\%USERNAME%\Documents\My Games\the-haunted-hells-reach\UDKGame\Config

(where %USERNAME% is your Windows login/profile name)
open UDKInput.ini file that is stored in that folder with Notepad (or any other text editor you prefer. I prefer Notepad++), press Ctrl+F and search for the word "Suicide".

You should find a line which will read like this:

Bindings=(Name="Z",Command="suicide",Control=False,Shift=False,Alt=False,bIgnoreCtrl=False,bIgnoreShift
=False,bIgnoreAlt=False)

Change the command (personally I just removed the letter "e" in the word "suicide" so it would turn into "suicid" and it worked for me.
Now save the file and close the editor. Viola - the job is done! No more random suicides on wave 3 or 4 just because you've accidentally pressed the wrong button on your keyboard!