Stop using random adventurers/noobs in green/low level party as power factor in lore. From Lore point of view it was special group who infiltrated DM and killed Vancleef in 1 against 5 fight. Besides like i said he fell to PC characters - that means because of gameplay factor no matter how strong you will fall.
By this gameplay logic we can assume Vrykyls in Northrend were more powerful than Nefarius in BWL since when we killed Nefarius we were adventurers and in WoTLK we knowns champions.