Paz will be unlocked at level 21, each level consists of 3 challenges which I will list below, most of these are self explanatory however I will add a note for any that are not.
For levels 1-10 see my guide for Maya here:
Maya is the best character for pretty much all of these challenges, she flips much faster than Alto, but doesn't build up speed as fast.
Level 11:
• Land 3 backflips with Maya in one run
• Land a 3x combo
• Pick up 50 coins with the coin magnet
Level 12:
• Collect 75 coins in one run
• Travel 2,000m in one run
• Jump off a wooden ramp onto a grind
Level 13:
• Land 2 double backflips in one run
• Survive a rainstorm - You can wait in the menu until a rainstorm starts to start your run if needed, a full day cycle takes 7 minutes 30 seconds, i'm unsure if a rainstorm is a daily occurrence though.
• Spook 30 birds in one run
Level 14:
• Jump across 3 chasms in one run
• Backflip onto a grind
• Get a grind bonus (60m without jumping)
Level 15:
• Backflip over a rock
• Catch 25 llamas in one run
• Rip 60 bunting flags in one run
Level 16:
• Catch a llama while hovering - May be useful to upgrade the timer on the hover feather if needed. If you don't have this by the time you get your triple backflip I suggest switching to Alto for this challenge as Maya tends to hover past the llamas.
• Land a triple backflip - Maya flips much faster than Alto, always use her when going for backflip challenges! Gather enough momentum and jump before a big drop, sometimes you will have enough time for a triple. Use your best judgement, it's better to land a double and try again at the next big drop than die and start again. The wingsuit will help alot with this challenge since you can use it to get higher and as soon as you finish the triple jump you can wingsuit to safety. Also you can bounce off rocks with your head, if you know you will land on a rock keep flipping!
• Bounce on a rock
Level 17:
• Travel 3,500m in one run
• Score 5,000 points in one run
• Grind 3 rooftops
Level 18:
• Grind to backflip to grind
• Land 3 double backflips in one run
• Slide on 5 ice patches
Level 19:
• Double backflip off a grind
• Jump off a wooden ramp onto a grind 3 times in one run
• Backflip over a campfire
Level 20:
• Land 2 triple backflips in one run - Same as in level 16, always use Maya, Wingsuit is amazeballs. Took me a while without the wingsuit but it is possible, just have to be patient and time it right.
• Land a 1,000 point combo
• Witness 3 lightning strikes - I assume this can be done by simply idling in the menu if you happen to finish the triple backflips before this one.
Once you complete Level 20 congratulations, you just unlocked Paz! Simply select him from the character screen located on the main menu and the achievement will pop.
Paz has longer sonic booms (for destroying rocks and campfires) and is faster once he gains momentum.