TY the Tasmanian Tiger

*Online required*
This achievement is a little bit buggy and also requires a friend to complete. Winning 50 challenges isn't difficult, but it will take time due to how the challenge system is set up on this game. For starters, know that you can only have one challenge out to a friend for each mini-game at once. This means you can not re-issue a challenge to a friend until they beat that specific challenge, so you can't just flood them with invites.

Now, the easiest way to get this is by sending challenges to a friend. For the danger arena matches, just simply run right, kill 1 frill, then run left into the approaching laser to end the challenge. Next, select the friend to send it to. Once they receive it, instruct them to fail the challenge by immediately running into the laser gate to the left without killing anything. (And vice versa when you receive their invites)

For the time trials, complete it as fast as you can, and make sure your friend who receives it completes it slower than you, for it to count as a win for you. (And vice versa when you receive their invites)

If you both followed that, then it should award you a trophy icon next to the challenge, and then subsequently remove it from your challenge notification, thus registering the win for you. If you both do it the opposite way, the system glitches, doesn't count your win, and leaves the challenge in your notification center where it thinks your opponent never completed the challenge. If that happens, just manually remove it by hitting the "x" next to it.

So, to clarify, make sure that your opponent loses the challenges they receive from you, and vice versa, to make this work.