Okay I did some math and jesus damn, OP is actually way right. So here are some maths. I took the liberty of calculating 3 different keystones for him. Go to the end to see the final result if that's all you're interested in.
First assumption is Brand took the following runes. They all seem fairly reasonable to take.
Summon Aery / Arcane Comet
Manaflow Band
Absolute Focus (100% hp)
Scorch
Cheap Shot
Eyeball Collection (0 Stacks)
Or
Electrocute
Cheap Shot
Eyeball Collection (0 Stacks)
Hunter Rune (Doesn't matter, none of them add damage)
Manaflow Band
Scorch
Another assumption is that his only items are a frostfang and amplifying tome (reasonable to have at level 6)
I assume Brand has been maxing W (And missed. Also missed his Q) So he has 1 point in E and R.
Last assumption is that the target has between 700 and 1000 hp. This is also reasonable around that level.
Here's the initial math. Brand's E + 3 bounces of R does 370 base magic damage (110% AP scaling) + 24.5% of the target's maximum health (1.5% AP scaling). At level 6, ignite does 205 true damage, cheap shot does 22 true damage, scorch does 32 magic damage, and Frostfang will add up to 54 magic damage. Also every non-melee enemy will have 32 magic resist, ASSUMING they're also level 6.
I would like to point out that at this point, we haven't even added AP or keystones to the equation, and we're at 700-770 post-mitigation damage. But hey let's go further.
Summon Aery = Between 834-897 total post-mitigation damage
Arcane Comet = Between 832-895 total post-mitigation damage
Electrocute = Between 901-963 total post-mitigation damage
Then add around 50 post mitigation physical damage for however many auto attacks he gets off. If you don't flash away it's probably at least two. So he can do around 1000 TOTAL POST-MITIGATION damage at level 6 with this build after whiffing his W and Q.

EDIT: Pretty late to edit this, but I realized Summon Aery will proc twice from the ER combo. So I added 30 damage appropriately.