After testing a range of gear options the only one that really made a difference was the Max Health – by using full Vigorous your companion should have 40.5k Max Health with Bastian or 39.4k Max Health with Mirri. Having higher health seemed to have a much bigger impact than anything else and also, for a Tank companion there are quite a few skills that scale on the companions Max Health so that helps also.
The damage reduction gear seemed to perform really poorly, combining different gear options didn’t do that great either compared with Vigorous.
Heavy Armor: 2900 Armor (per piece)
Vigorous Trait: Increases Max Health by 2.6% (per piece)
There are a variety of skills you can use for your companions. The priority for a Tank companion is having skills with a high uptime and low cooldown that provide high survivability. You have to really look at the skills that have a low cooldown time or high uptime, especially from a Tanking perspective you want them to be using survival skills as often as possible.
To maximise their capability to Tank you want to use damage reduction, shield and self healing skills mainly. If you are healing your companion then you could switch to using some crowd control skills such as a gap closer/pull/immobilise/fear skills that are available, the problem with these skills are the cooldowns. In an add pull with 4 ranged enemies and a chain cooldown of 8 seconds, your companion can never pull them in before you can just run around and kill them.
Best Tank Companion
Bastian & Isobel are much better at dealing with damage situations compared to Mirri & Ember. Both Bastian and Isobel can keep themselves alive in most situations that they were tested in with this build including World Bosses, Delves, Overland Enemies, Normal Dungeons, Non-DLC Vet Dungeons, Normal Trials.