Bog boss in Wailing Caverns

Yesterday I ran Wailing Caverns on my priest and died at level 21 to the bog boss. The tank charged in with a 2h and got absolutely stomped by the boss, in 2 hits he was at 30%hp and switched to a shield. I casted a big heal and instantly got agro, he ran to me, first hit removed my shield and 40% of my hp, 2nd hit killed me. What am I supposed to do here? If I don't use a big heal the tank dies and if I use a big heal I get agro and die.