I recommend resto staff, because melee companions die a lot more often as far as I can tell.

Companions seem to take less AoE damage from certain attacks (like sorc pets), but it's not clear how well that works. Resto staff Bastian has survived several dragon fights for me. For him to tank dragons specifically, if that's even possible, you might need a different build. In fact, different bosses might flat-out require slightly different builds, but this is my general suggestion for single-target tanking.

This is the skill order I would recommend, but haven't tested all these skills out

1. Mystic Fortress (resto staff)

25k damage shield every time you or Bastian get under 25% health. Pretty clutch.

2. Drake's Blood (draconic armor)

Big burst heal and reduces damage taken.

3. Savage Instinct (undaunted)

Bastian's taunt.

4-5. Rejuvenation, Reverse Entropy, Basalt Barrier (any 2 of these)

Having heals-over-time increases my Bastian's lifespan by a lot. Rejuvenation targets both you and Bastian every time—not sure about Entropy.

My problem with Cauterize is that it doesn't heal for very much and only activates under 75% health, but maybe it's good paired with a HoT.

Good luck, let us know what does and doesn't work!

Agree with @Starlight_Whisper . Once you have those three skills and get room for a fourth, I like to add Bashing Bulwark as a gap closer stun.

I only use companion tank if the character I'm using has reasonably strong 'heal other' skills and is prepared to keep an eye on the tank. Even a basic one like the larger radius Vigor should do the trick on a stam char.

Commanding the tank to go in first, then supporting with heals and dps works very well.

Thank you so so much all of you I redid him this morning based on this. Very much appreciated