Earth/Fire Elementalist... with some secondary profession - I don't know which one would be good...
Obsidian Flesh (casted first - prevent any enemy spells)
Ward against melee (50% chance evade melee)
Magnetic aura (75% chance block melee)
Inferno (15-99 damage around you)
Phoenix (22-166 damage around you)
Rodgort's Invocation (7-91 damage and burn for 1-3 seconds)
Meteor Shower (7-91 damage per wave, with knockdown)
That leaves one skill open for maybe a secondary profession to help out? Maybe arcane echo with a mesmer? You'd run into the middle of a crowd, cast those 3 earth spells with maybe a few groups of melee'ers whacking at you, then you just spam those 4 fire spells and everything dies. Would it work? If not, why not - and what can I do to improve this build?