Dredge fractal
You just said it yourself. One person aggros the dredges, but he has to do it right. The person doesn’t just aggro the dredges and stand there, they have to aggro then run back to make the dredges move away from the wall. It helps if you have a thief also.
With the thief the person that aggros start aggroing all the dredges and move back. Don’t plant the bombs until the dredges move away from the wall. Planting the bombs before the dredges move away doesn’t work.