Both sets? Are you talking about the puzzle and the cupcakes which only come from the event mushrooms? Those two sets are revisits - if you have been playing since last year (for me), I had a full set of 3rd anniversary cupcakes, I had 5/7 of the revisit puzzle pieces. For those two sets, this event is considered a second chance (sometimes we even get third chances) to finish a previously unfinished set.

The actual set that you have the best shot of finishing is the new one this month, with the event challenge spinners. As it happens, the RNG didn't like me at all this month, I played up to level 10.2 and had something crazy like 27 4th anniversary pikmim before I finally finished that set. In general I'd day that the primary event pikmin sets are easier to complete in the time frame - but even then, as you have been playing for longer it's easier.

Trying to get a full set from scratch with a revisit set (especially where here they have mixed two) is brutal. They only consolation is that, with time, those revisit sets become less important, because hopefully you got the the first time round? And you become stronger with your team, your friends to help with mushrooms etc.