I'm assuming he was kiting them and not dps them at meele range . If so, tell him to tank them normally , and just run to behind 2 of them when they both start casting their aoe .
The aoe is a cone attack and it will not change its direction once started casting .
Also, what melted him is the dot i think , having one dps able to cleanse it helps greatly ,otherwise just blow all cd (not at the same time) and kill them before they kill you.