I can do T4 solo, but playing with my Monk friend who is used to pull allot of packs i tend to be more often dead then alive or have to run away. Gargantuan doesn't 1-shot things anymore and one have to stand there and do sustainable damage to get those billions of hp points down from elites.
Perhaps it was intended by blizzard to make Pets Uber in T1-3 (my humble opinion) or are ppl actually soloing T6 with pet/firebats build successfully?
If you have enough + damage with jarem, attack speed for pets gloves, and cindercoat, magefist, soj, ammy, and bracers with bonus fire. gargantuan will clean packs up for you solo in t5, do significant damage in t6. If you are in a party, just think of the garg as a nuke. he is hitting for 200-300m pretty regularly. My issue with the fire build is, everything else kind of blows. Fetish army is great, but physical fetish are better for a number of reasons, the least of witch legion of daggers gives 8 physical fetish that NEVER bug out. Tiki and the Dart fetishes stop attacking sometimes. So fire builds are viable, you will do fine in t5/6 with the right gear, but I personally think both Poison and Physical are better.
It's very possible to clear t6 as a petdoc.I'm personally a fire-petdoc and my Gigantium crits for 300mil++. Even t6 packs dont survive long against that. Survivability wise I'm fine in most t6 rifts. The only issue I have are the very stupit rifts with a ton of act5 mobs (death maidens,barbed lurkers, anarchs etc). Those can be somewhat problematic, but if I swap my stone of jordan for my unity im fine again.