How do I unlock level 25 of Far Future?

Solution 1:

In previous versions, the final level was available for each world as soon as you unlocked it. Then they made some big changes to the game including keeping level 25 (the boss fight) locked for each world. Each world's level 25 is unlocked as part of an update.

IMO this mechanic is an attempt to prolong interest in the game without adding more content. This mechanic was not conveyed well, especially for people who'd already completed a world only to find it now saying 24/25 complete with level 25 mysteriously unavailable.

I assume Far Future will work the same way. Its level 25 will be unlocked for everyone in some future update.