You van Connect a Power up to enemy and when he Dies and Drop power up so you collect it. You put a event observer in the scene where when you collect X power ups so you will be redirected to next scene. So you Create the same number of enemies and power ups connected each other.
What i would do is say you have 30 enemies. Add an ACTION coin to game. Drop it in scene and then COPY it. control C on pc. Open up enemy menu on left said and go into its defeated animation setting, hit control V to paste the coin in. BLOW UP the coin inside the enemies defeated animation to 40 / 40 and make its opacity 0. Now, when each enemy dies, an invisible large coin hits player. You now have 1 coin. Now go to WORLD UI and set it so it takes 30 session coins to win level. Done. Now every time you kill an enemy, you gain a coin and reach the goal.