I Am Thrall's Complete Lack Of Surprise

To access random brawl mode, buy [item=143763] from [npc=118898]

I personally tested this myself today to see if there is a new achievement this time around.

I killed 34 total bosses using the "Bag of Chipped Dice" inside the random brawl mode.

23 of the bosses were the same that I beat during the Rank 1 to Rank 7 fights.

11 were the bosses tied to the challenge cards or on the "The Best There Is" achievement list.

Kirrawk, Nibbleh, Fran and Riddoh, Dominika the Illusionist, Proboskus, Thog Hammerspace, Dungeon Master Vishas, Blind Hero, Vian the Volatile, Millie Watt, and Fjoll.

So I can confirm there is no new achievement.
I cannot confirm if the old achievement is still obtainable, as I already have those two.

If you'd like the full Brawler's Guild guide I'll have it uploaded in the video section of this item soon!

In 11.2.7 you only need to defeat 10 random bosses, Even if they are ranked or challenge ones defeating ANY 10 at random will count towards the achievement, I can also confirm that you will get both achievements for doing so if you have not unlocked the old one