Ghoulfiends and bonuses

[yoho]

Hello. I ran into this old topic but I'd like to clarify further.

We played yesterday the very basic Muchkin and one of us being already on level 9 got "Ghoulfiends" when opened a Door. Another player said that you can modify Ghoulfiends with a card like "Enraged". The first player said: No you can't it's level only and no bonuses but "Pollymorph" or other cards which "deny" monster are allowed.

The problem is, we're all Russians and we play russian deck. And russian card says, literally: "No bonuses will help you in battle against them. Defeat them with greatness of your level". In english I see: "No items or other bonuses help against them – fight with your Level only"

So could you please clarify how exactly this works? Do I understand it correctly that bonuses do not help against Ghoulfiends only (but can help them)?
And... what are those "bonuses" mentioned exactly?
* Is Warrior or Thief ability is a bonus I cannot use (I mean, get stronger as Warrior or make fighting player weaker as a thief)?
* Is potion of invisibility or pollymorph a bonus I cannot use?
* Is a card like "Enraged" a bonus I cannot use?
* is a card like "Freezing Explosive Potion" or "Pretty Balloons" a bonus I cannot use?
* does this rule still work when I throw a dice trying to flee from battle?

Thank you