Specifically, we are looking for the rare spawn of the End of Chapter bosses. Here's a breakdown of the bosses and their possible spawns per chapter:

Chapter 1
• Gurag the Destroyer - Common spawn
  
• Gurag the Demolisher - Uncommon spawn
  
• Gurag the Demi-God - Rare spawn

Chapter 2
• Grim Reaper - Common spawn
  
• Soul Harvester - Uncommon spawn✝
  
• Death - Rare spawn

Chapter 3*
• Damien - Common spawn
  
• Wretched Kid - Uncommon spawn
  
• Son of Lucifer - Rare Spawn

Chapter 4
• Anubis - Common spawn
  
• Guardian of the Eternal Life - Uncommon spawn
  
• Reincarnation of Ramses II - Rare Spawn

Chapter 5 **
• Karp of Doom - Common spawn
  
• Aku - Uncommon spawn
  
• Gaforashyu - Rare spawn

If you're ever unsure and in the heat of the moment and don't remember, the common spawn has a basic healthbar, the uncommon one has a grey winged health bar, and the rare spawn has a golden health bar.

*In chapter 3, the final boss will not have a unique modifier to its health bar
** In chapter 5, this exploit will not work, if you attempt to do this, you will be moved to wave 1, chapter 1. (but as wave 150, chapter 1 in which your game will be bugged. To remove the bug, save and quit and enter a shop, this resets the 'boss wave' and you can continue as normal. Unfortunately, you will have to do every wave and chapter again)
✝Credit to _s445_ for that

What we will do, is exploit the games save system. Quite simply. Once you clear a throne dungeon (the chapter end boss' lair) the boss will die and an item, money, or six chest will spawn in addition to a portal.

However, there is no need to go through the portal.

Instead, collect what you want and then save and quit and proceed to open your character. Using this method it is possible to farm boss spawns, items, money, keys, potions and levels.

For example, if you wanted to, you can keep doing Gurags dungeon (or any boss for the matter) again and again until you have collected every single relic in the game or are satisfied with your grind.

It is through this method we will eliminate the need to play full playthroughs to get to the specific boss you need to get to.

There is a bonus however, an edge, for those with low performance PCs or get significant lag in throne dungeons. If you can achieve a framerate of 3fps, you can pass through any fence blocking your way. Thus potentially skipping both Damiens and Deaths dungeons while you make a beeline for the boss.

Conquer
Save
Repeat
Profit.

There are a couple of other uses for save exploitation that can be used at any time to achieve a number of things such as the following:

Don't like your spawn? Save and restart the wave.
Want more chest when you load in? Save and restart the wave.
Having issues finding the levers for activating a statue? Save, rinse, repeat.

Frankly, this is a huge exploit that puts a giant hole into the whole "rouge-like" element, because you can farm anything you want, at any time.

I'm sure this will get patched at some point, but, I do recommend trying to play a run to completion before using these methods, because, afterall, you wouldn't want to spoil the challenge (or unbalanced) difficulty.

As far as I can tell, the game, for whatever reason ALWAYS saves whatever you pickup in a wave as soon as you save. But, it WILL NOT save the completion of a throne dungeon until you move on to the next wave. It is because of this that the exploitable farming is possible.


Have a great day and have fun.