If I remember correctly, you need to wait for Karkinos to charge, and then roll the eggs into its back, knocking it over.
Also, a tip for later in the game: weapons with health regeneration on hit are among the best in the game. The health recovery is equal to a percentage of your damage, so possessed scythes with health regen, critical chance and critical damage can get you far, and make the game's relatively high enemy damage values far more manageable.
My possessed scythes built this way got me through the first 95 waves of the Crucible in one sitting. Then I died to a Sorrow.