I was doing some exercises and I foud the sentence "er weiß gut Bescheid", I thougt I should say "er weiß guten Bescheid", because it's a stark declension, is that correct?
No, despite looking similar, this is not the construction as in
Er weiß guten Rat (He has good advice)
but an adverb stating the degree of his knowledge. So the meaning of gut is the same as in
Ich kenne ihn gut. (I know him well).
So the translation is actually:
He is well acquainted with that topic.