*Freeze water in a loaf pan or similar container. It'll keep the punch cold without diluting the mixture.
For lemon shrub
Zest of 8-10 lemons
1 pound demerara sugar
17 ounces lemon juice
Combine ingredients in a punch bowl over the ice block, stir to combine, and garnish with lemon slices
For lemon shrub:
Muddle zest and sugar to release essential lemon oils—this is known as an oleosac-charum. By hand, the process can take up to 1 hour. To expedite the process, use a stand mixer. With a paddle attachment, beat zest and sugar for 15 minutes, then let rest 5 to 10 minutes. Using a whisk attachment on lowest speed, add lemon juice to the oleo saccharum and mix for 10 minutes to dissolve all the sugar. Pass through a fine strainer to remove zest, then bottle mixture.