I've been running an Incinerate Witch right up to 78 and its worked out pretty nicely, only real issue was when it tried to target stuff outside of my effective Incinerate range however after picking up +30% to projectile range via passives and gem quality that's no longer an issue. To be honest I find it the safest farming build and far quicker than my Infernal blow Mar considering it takes almost no gear and only improves when you start grabbing key parts (Infernal mantle, Saff shield). My Mar has really struggled in terms of DPS / Survivability due to my poor luck getting decent gear for him.
Reflect isn't an issue, with the right gear neither is thorns, Great Multi allows you to fry a whole room fairly nicely without to much need to target anything. And to top it off it has great tanking potential with mind of matter/EB and a bit of armour and Leech.