Combine lime-flavored gelatin mix and boiling water in a bowl; stir until gelatin is dissolved. Stir 1 cup tequila into gelatin mixture. Pour into about 24 shot glasses and place in refrigerator to set, at least 1 hour.
Salt the rims of 4 to 6 martini glasses.
Combine ice, margarita mix, and 1/2 cup tequila in a cocktail shaker; shake until well mixed. Place a lime-flavored gelatin shot in the bottom of each prepared martini glass. Pour margarita mixture over the shot, leaving enough room to top with beer. Top each margarita-martini with beer. Garnish with a lime slice.