How the combo works:
Reduce the mana cost of Knuckles by 1 mana with Helpless Hatchling then try to buff Knuckles with Hidden Cache. Note: only use Helpless Hatchling when having Subject 9 played and only use Hidden Cache when having both Helpless Hatchling and Subject 9 played.
Now that your Knuckles costs 4 mana and may have been buffed by Hidden Cache you can use the full combo with 10 mana. Knuckles–>2xGoblin Prank–>2xThe Beast Within and then attack with it. This order will maximise the damage you can do.
Preferably you want 3 minions on the opponents board and hit 2 of them with the random attack from The Beast Within and the third attack with your own “decision”