You will encounter Workers almost immediately upon setting foot in the Hive, the 3rd level.
They will conjure a yellowish ball of goop from their backs then hurl it towards you.
You can either "catch" the gunk with your shield and shoot it back, or just flash your shield at the last second to reflect it back on the same path it came.
The early part of the Hive can be a bit tough when you first enter; if you find yourself dying before you can rack up 10 of these, try waiting until after the "toad" mini-boss at which point you will encounter several Hive Workers in short order.