For templar dps setting Bastian as a tank would work nicely because you can also heal him via your skills, and if you fight melee too, you can more easily get the synergies of some of his skills. When playing a templar tank you can set either Mirri or Ember as a melee dps, I think Mirri would work better in melee range, and she can CC add packs as well.

i have isobel set up as my tank companion right now

using provoke (1h/shield), her templar class shield, on guard (1h/shield), and then her breath of life equivalent and purifying light equivalent, she can survive most things fairly well (between the 2 shields she gets like 37.5% max hp as dmg shield and 20% dmg reduction) and 2 heals (i explicitly unslotted her awful ultimate too, which has a near 4 sec cast time)

i usually pair her with my templar dps to run solo dungeons as she can tank most things, with just the support of the healing from the cleansing ritual

for my tank characters, i usually use ember as the dps (though i have mirri also set up as a dps, i think ember is a bit better)

for embers dps spec i have her using starfall (companions mages guild), her crystal blast, her sorc shield (which also heals), entomb (her sorc immob, which also heals her), and her sorc skill which reduces the cooldown of her other skills (so she can more frequently use her bursty dmg skills, and keep her shields/heal over times up pretty much all the time)

i also prefer her ultimate because its basically instant cast (while the other companions ults have very long cast times)

I find Mirri makes the best tank due to her brief disappear and heal skill which makes her very durable and, if she loses aggro while invis, she gets it back very quickly. I also quite like the crowd control that her fear spell provides.

I find Ember makes the next best tank due to her healing skill, shielding skill and crowd control from shards.