Endless Hard mode only unlocks when you've reached 100 on Endless mode.
To earn this achievement, simply use the method explained in the Endless Score: One Hundred achievement:
Put the game into windowed mode and then run Endless mode. If you're unsure of an answer, right-click on the top of the game's window which will effectively pause the game. Whilst it's paused you can use another device such as a phone, tablet or laptop to look up the answer via Google. Unfortunately, if you click anywhere else using the computer you're using to play this game, it will unpause.