conjurer is always my goto recommend for simple HC pets
Could do Blightfiend-Briar ritualist, but tbh the occultist heal/Blood of Dreeg sorta makes conjurer way more chill levelling imo, and Necro would carry couple more buttons anyway.
for levelling tips we have this 🔰 Beginner Build Compendium - 126 guides for AoM & FG expansions
Maya has made some extra chonky builds for endgame, not fastest killers on the planet, but very safe and comfy imo (got me my first HC superboss kill for FG back in the day) The Carnival - A Guide to Pets
no, pets need resists just like player, or they take extra dmg and you end up having to need to resummon them more, which can end up detrimental if they start taking so much dmg from such low resist they die before cooldown is ready. So one should def glance at the pet stat sheet in page 2 once in a while