As far as platinum companions go, The Brave is one of the simpler, more straightforward ones in the game. No Brimstone encounter needs to be conquered, although the finale does require some advanced preparation and skill.
Inspect Hubie's character card for some very obvious clues as to what his desired encounters are when you reach that stage of the questline.
Soon you will arrive at a certain platinum encounter card:
A "choose your adventure" style story plays out, which grants a token leading to the final encounter. You will fight 3 consecutive battles against Blight creatures and spires. You will take 3 separate Brimstone life pain cards for 30 damage each in between the skirmishes. Meaning, unless you're untouchable in combat, raising your max life beforehand would be wise. I began the encounter at 120/120 life and barely survived with only 3 health remaining. I also was lucky enough to have Pious Robes equipped, which reduces the damage caused by Corrupted foes.
Survive this, and Hubie's platinum card will be yours. It is possibly the most coveted achievement of all the DLC for its high gamerscore and ratio. After netting this, using your new campfire ability 10 times nabs you an additional easy 15 gamerscore (see below):
The New Horn achievement in Hand of Fate 2 worth 58 pointsUse Hubie's Portal ability at the campfire 10 times.