As you run around the overworld, you will notice small bottles that look like potions. These are tonics.
As you play through the story, you will be given one for free. Of course, this achievement requires you to have two. So, you now have to have two things:
750 quills
1 tonic you have picked up from the overworld
Once you have both of these, press the
and hit
until you get to the tonics page. You can now purchase the second tonic. Select it and equip it in the second tonic slot. Make sure you have your free one equipped at the same time.
Now head to any level, complete it and the achievement should unlock!