you could rent tiny amount of SHA256 hashrate
What amount of hashrate do you consider, tiny, but powerful enough to mine ~100 coins in not more than few days?
(Btw what is the current block reward on testnet?)
Current block reward : 0.39162548 tBTC
Current difficult : 13494542.63853603
Current network hashrate : 65827412001037.67
With the mining calculator help from
https://www.cryptoground.com/bitcoin-mining-calculator, you'd need about 100 TH/s (0.1 PH/s) where you'd able to get 58.3803 tBTC/day.
After calculation process and compare it with
https://www.nicehash.com/buy, roughly you need to spend $60 - $70. Also, 100 TH/s is 1.5x from current total testnet hashrate.
It's far bigger than i thought and clearly this isn't good solution.