Here's the quick breakdown: Level 7 is where this buddy really comes to life, so level 8 is a great example.

This build uses a Human Primal Companion Hunter. So you start with a cuddly medium sized T-Rex, who grows to large size at 7th level. His bite is really the pride and joy of this build. Powerful build lets you add double your strength bonus to damage. So we're looking at maximizing damage by both maximizing strength bonus, and base damage dice. I'll take both separately. Primal Companion gets you some nice tricks to bump both. I'm going to start with the evolutions that help supercharge your pet chomper.

Large: Your T-Rex is already large, but at 8th level the evolution states that you can "spend 6 evolution points to become huge". You have 11 to play with, so with another 1 you get the "increased damage" evolution. After that you can pick whatever you want. If your DM is super strict and makes you pay for all 10 points of the large mutation that's all you can afford.

For base damage your chomper has 2d6 when large. The evolution increases you to huge, getting 3d6 based damage. Your critter can take the improved natural attack feat, for an increase to 4d6. Finally, to round everything out you can cast Strong Jaw as a 3rd level spell (yay ranger list!), so that doubles your damage up to a modest 8d6 per chomp.

Now, for the strength bonus. 14 Base, +16 for size, +2 companion bonus +2 Eye for Talent. That's a strength of 34, bonus of +12, so a +24 damage bonus. Throw on power attack and you'll be up to a cool +30.

So now your buddy at level 8 can chomp away as a huge T-Rex with 10-15 foot reach (depending on how you read it), at +13/+8 to hit for 8d6+30 damage. Keep in mind this is with only the primal transformation power and strong jaw active, so anything else you want to do for buffs should be fair game. That's also with zero magic gear. Plenty of room for improvement, this is just the base chompy chasis, if you will.