For those of you not cheating though the story, or even those just looking to give Boss Rush a go; here is a basic guide for each of the Dungeon Bosses:
Spoiler Free.
V0.5
Most of the bosses seem to have been designed with the parry mechanic in mind, which may take a little practice.
BOSS GUIDE
The Witch:
Use a melee weapon, whichever you're the most comfortable swinging, but ideally the Katana.
Stay fairly close to her. She'll periodically dodge sideways, multiple times in a row, so being point blank can make her awkward to track. The opposite to this being, don't be too far.
All her attacks are fire projectiles; a simple parry, returns to sender. Being too far will allow her time to move before the projectile reaches her. If you're close enough, the return will hit and stun her.
Either sneak in a few other weapon attacks or just keep swinging.
Repeat until defeated.
The Duchess:
There are TWO ways to fight her. 1) The standard way, how the fight was designed. 2) A faster, cheesier way.
Both require a ranged weapon, as this fight would take ages if you only made melee strikes.
1).Ignore her for the first part of this fight, her shield makes her invulnerable to ranged attacks. Destroy her minions with whatever weapon you choose.
Defeating the last minion pops the shield and stuns her. I found the Thompson a great weapon here, for the run and gun, but any ranged weapon will do.
She will attempt to run, and is vulnerable to damage the whole time. However, do not get too close. The Duchess has a stunning shove that will give her room and time. She will make two telegraphed and predictable runs around parts of the room before resummoning minions and recharging the shield. There is a small window after her summons where her shield isn't back up. Exploit this if she is on low health.
Repeat until defeated.
2). Ignore her summons and immediately run her down with a melee weapon. She will automatically stun you, but strike with your weapon as you're stunned.
Her shield will pop.
Continue running her down and attacking her with a ranged weapon. Repeating the melee attack to pop the shield as required.
As with the first way, she will run two telegraphed routes before recharging the shield. You can damage her every step of the way, but she will automatically stun you if you get too close.
Repeat until defeated.
The Slacker:
Again, another boss where the parry mechanic will save you time, pain, and ammo.
This fight is in a room with large spike traps. These are pretty much for show and can easily be avoided and ignored. The slacker is highly aggressive in attacks as she is a melee opponent.
The Slacker also has a shield, but it can easily be popped by a melee attack.
Parry her to create stuns and easy openings. She is fast with her attack patterns, so parry earlier than you think.
After each shield is popped she will darken the screen, drop bear traps, and run into the dark. Move to a new area to fight her without needing to pay attention to your feet and wait for her to come back.
OR... If you're fast enough; as she turns to run, keep pace with her and continue damaging her as she runs. This in turn will also provide a new area to fight, away from her bear traps.
Repeat until defeated.
The Girlfriend:
This Bitch... Will fire at you the moment you spawn.
But really, this fight is... uh... Bad. (Love you Krasue).
It's the second fight where a ranged weapon is advised.
It's such a slog, it takes the longest out of the Dungeon Bosses to to Rush. Yet, if you have a high rate of fire weapon like the Thompson, and maxed out Leech stat, you basically lose no health. You'll risk breaking your weapon more than anything.
But if she is indeed causing you damage, get a pillar between you both and take shots at her, then take cover whilst reloading.
Bait her into throwing her grenades at the pillar if you can, as this will stun her if they bounce back/nearby, this is awkward to pull-off though.
If you're having a lot of trouble, she likewise will reload periodically, allowing you to damage her without retaliation for a couple of seconds.
The Girlfriend will seem to fire around corners at times, so you may need to reposition, but ultimately, this is just a gun fight with a VERY spongey boss.
Repeat until defeated.
The Mangler:
!!!If you are fighting her for the first time in the game, go into this fight OVER LEVELLED and OVER EQUIPPED.
Think you're all set? Good, now go back and make another Dungeon run for some more currency.
(If you've legitimately beat the game, you know why)!!!
The Mangler caught me off guard.
Take a melee weapon, be warmed up, and be practiced on your parrying.
This Boss is more aggressive than the Slacker. She's also very fast with a dodge that she will use to close the gap, or periodically side dodge.
Other than that, she's not in any way special. Parry her, stun her, and beat her ass. All without really leaving the same spot, as she'll only dodge around you once, meaning she's easier to track than The Witch.
You can always run around the room and shoot her, but her dodge makes this extremely annoying and not worth it, as she closes the gap after a couple shots.
Repeat until defeated.
!!!Then have your world flipped upside down. I did warn you!!!