Step 1. Dont complete any reward-islands unless its a boss
Step 2. Get 19 reward-islands on your board, where none of them is a boss.
Step 3. Run a +79 zone and get a building block.
Step 4. Place the +79 building block. There is a chance to spawn reward-islands any time you place a building block. 20 reward-islands is max so the 20th will always spawn as a boss-room if there are no other boss-islands on your board (level 79+ makes endgame boss spawnable). You now have 1/5 chance to spawn the leauge end-boss, as there are 5 bosses.
Step 5. You go to quarry and run shit zones to gather enough building blocks to get to the bossroom. Kill boss. Now you are back down to 19 reward-islands.
Step 6. See step 3 for unlimited boss-encounters.