From what I see, the best strategy is to save enough to get the 1st place. Due to the game nature, if you are the 1st to reach max point in this case 20k, no body can overtake you later.
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unless they keep opening chest even without rewards from the event
Edited
1. you need minimum 3660 Points to reach 5000 (the 1st round)
2. you need minimum 7260 Points to reach 10000 (the 2nd round)
3. you need minimum 10860 Points to reach 15000 (the 3rd round)
來源
I didn't understand