Very good and interesting question, because indeed the order of killing these bosses has a decisive influence on the success or failure during the quest.
To determine the order of fights with bosses you need to take into account these factors:
- difficulty of accessing
- difficulty of fighting the boss
- taints - kind of a curse you get after defeating the boss (especially 1st - 10% chance, that monster can teleport next to you and 3rd - getting 15% more damage)
I propose to do them in the following order:
1. Claustrophobic Inferno (Goshnar's Malice
)
To gain access, we have to survive 3 waves of monsters (mainly Brachiodemons + few Infernal Demons), standing in the corner it seems relatively easy, especially without any taints.
But the reason, why this mini boss is usually chosen as the first one is that - Goshnar's Malice is problably the most dangerous boss for Elite Knight and the most "punishing" at the time of the mistake.
You have to fully focus on killing the cage with the boss, otherwise the boss gains immunity and we won't be able to kill him (often there is barely enough time). The ground explodes in the room from time to time, which deals about 5-8k damage if we are not standing on a white SQMs. Having Stone Skin Amulet and Might Ring on lowers this damage a lot, but the summons tend to remove the SSA very fast, so you can be unlucky.
2. Mirrored Nightmare (Goshnar's Greed 
)
To gain access we have to fight with Mirrored Images. In the spooky version of Thais city there are a lot of monsters (Mirrored Images and Many Faces) so it's hard to kill one at the time. It's also very important to have 4 vocations team or at least take someone with you to help you finish the access (as a 6th, 7th player in team).
In case of this boss it is very important to have big damage. Worth to mention that Goshnar's Greed strong self healing (lifeleech) makes everything harder. On the other hand his helpers - Greedbeasts hits very strongly (about 5k). It all takes time. Fighting with him with few taints excludes the possibility of making mistakes, e.g. luring Weak Souls to the portals, due to lack of time.
3. Ebb and Flow (Goshnar's Spite 
)
The boss' mechanics is quite easy. Theoretically the third boss on this list may be Goshnar's Cruelty, because both are on a very similar level of difficulty, but due to the fact that Goshnar's Spite has more summons and the acces is really hard and dangerous (because of the new "fear" mechanics) I suggest you this order.
4. Furious Crater (Goshnar's Cruelty
)
Another quite easy boss, with more dangerous access. While the mechanics of the boss is really clear and easy then the way to the boss might be dangerous even for high level players. To make an access and be able to go on a deeper level of Furious Crater you have to collect Pulsating Energies 
(particular amount for each level), but when you change levels you suddenly appear on a completely new area with a lot of monsters around. The deeper you are, the smaller area is around you and it's hard to not lure too many creatures.
5. Rotten Wasteland (Goshnar's Hatred
)
To make access you have to activate a portal by using four different shrines (Blue, Green, Orange and Violet) - let me say it is easy. What is more, the fight with Goshnar's Hatred is the easiest one. Most of damage is dealt by summons of the boss, but mages use mainly Great Fireball Runes, which helps to reduce summons' appearing.
Since access as well as fight with the mini boss are the easiest - that's why you should do this in the end, with 4 taints.
Now it's time to fight with the main boss - Goshnar's Megalomania 
!
Hope it helps!
Source of screenshots: https://tibiopedia.pl/quests/Soul_War_Quest
Worth to add some spoilers to the Soul War Quest:
TibiaQA.com is a fansite. Please note that the only official website is Tibia.com. The game Tibia and the website Tibia.com are copyrighted by CipSoft GmbH.
