In a blender of food processor, puree the watermelon until smooth. Pour through a strainer to remove the froth - you will end up with about 1/2-3/4 cup liquid.
In a shallow bowl or on a plate, whisk together sugar, chili powder, and lime zest.
Swipe a lime wedge around the rims of two cocktail glasses. Dip and swirl the rims in the chili lime sugar. Fill each glass with ice.
Muddle mint leaves and lime juice in a cocktail shaker. Measure in agave, 1/2 cup watermelon puree, mezcal, and tequila. Add ice and shake until icy cold. Strain evenly into both glasses. Garnish with mint leaves and watermelon triangles. Ole!