I found them rather easy to kill with a solo tamer. What helps me probably is that I am coming from a dexxer background and there you tailor your suit and weapon specifically for each monster type you wish to fight. The same applies to tamers and their pet builds but most tamers simply use one pet for everything in the game and when they realize that this specific pet won't work for a specific type of monster they conclude that this monster is impossible to kill for tamers.
@North_LS what you experienced is a blackthorn captain with the consume damage ability. they will use all physical dmg dealt to them to heal themselves which is why after 10 minutes you still didn't see any damage. not all blackthorn captains spawn with this ability but enough to make pets with 100% physical damage inconvenient down there
solution: bring a pet with 100% elemental damage (anything but physical will do)
I've successfully used fire beetles, cu sidhes, giant beetles and lesser hiryus down there (giant beetles and lesser hiryus obviously only work against the regular captains without the consume dmg ability). I strongly recommend bringing pets with all skills at 120. I am pretty sure it can be done with pets that only ate 110s or so but it will take significantly longer to kill a captain that way.
For the melee type captains I use consume to keep the pet healed.
For the caster type captains it's usually enough to cast greater heals and cures.
Having 120 spellweaving (on the tamer) and a str 6 focus can be really helpful to speed things up towards the end, especially with captains that like to heal themselves constantly.
each of the captains has a special move that requires you to adjust your behavior. some of them like to pull you in and switch targets, others have area attacks or nasty debuffs. You need reliable ways to heal, cure and invis yourself to survive when they target you.
none of the captains can be discorded, so bring more usefull skills than bard skills.