Classic mode can be found by going to Single Player -> Freeplay from the main menu. Surviving 45 seconds shouldn't be too difficult. I was able to earn this achievement on my second attempt. If you're struggling and have access to a Windows 10 PC, this achievement might be easier using a mouse.
You should also make sure that all modifiers are turned off, as using any modifiers will void the achievement (thanks cwing302
!). The default settings for Classic mode have no modifiers enabled, so as long as you don't enable any modifiers, you should be good to go.