Mundus is a boss in Mission 19 and has two phases. First phase is breaking his arms then his core. Players usually lose massive amount of time at this segement due to:
Changes to the original game
The arms will do several different types of attacks that often stay on top of the building giving you a chance to damage them. Definitve Edition can break them with DT Judgement spam. Original game must use Demon Evade before DT Judgement because his health pool in that version is much larger. As a result, the timing of each cycle is much more stricter.
When Mundus performs Lava Breath. Fall off from the ledge to force him into a different pattern. If you do it too fast then he will keep doing the same pattern. Very common in phase 2.
Deflecting Meteor Rain is a lot harder in the Original game because it requires 2 out of 3 meteors compared to Definitve Edition which is only once. Most reliable way to do it is 1 Reb strike into Osiris Feed.
Original game demo
Definitve Edition demo