You gain 1 coin for every 100 points you get in a level. That means you need to get 10 million points for 100,000 coins. This will be the last achievement you will get during your Endless mode gameplay.
Thanks to neeker75
for sharing this tip:
"The grinding achievements (amassing coins, dropping/ destroying X number of blocks) can be unlocked at one go by putting in the x2, lateral addition and hints boosts and playing endless/ infinite mode. Just keep putting the blocks where the game tells you to (with hints turned on) until all the grinding is out of the way."
We are looking at hours of grinding here so I suggest taking breaks and just play the game every day for about 30 minutes or how much you feel like doing. You can back out of the game with your phones Back button and close the game. When you next time open up the Endless game mode, it will continue where you left it last time. Good luck!