If you are trying to defend Catholic dogma, it can't be done from scripture. Firstly, you are mistaken about the role of a mediator and that of angel. The very words for angel both in the Old and New Testaments mean "messenger" and not a mediator. A mediator has the power to negotiate between two parties and often, as in government-legislated mediation, his decisions are final. The messenger who brings a telegram to the mediation conference room is NOT a mediator, he is a messenger. The angel who brought the news of Jesus' birth to Mary was a messenger, he wasn't mediating anything. There was no negotiation at all. What really puts paid to your argument is this passage of scripture:
1 Timothy 2:5 There is one God and one mediator between God and man, the man Christ Jesus.
The Bible clearly states that there is one and ONLY one mediator between God and men. End of story. You can rely on the "Magesterium" of the church if you like, I'll stick with the Bible.
Here is a link to information regarding your question:
Mary, the Mother of Jesus: https://www.thisisyourbible.com/library/show/1035
I hope you have found this helpful.
God bless you,