Beat all the ultimate challenges and obtained 100% completion. Now I can finally say I've thoroughly played this game start to finish at least once, justifying a 5+ year purchase.

UC-1 and UC-4 weren't anywhere as difficult as UC-2, the toad escort level took awhile but it was a breeze mostly. UC-4 needed one retry but otherwise relatively easy.

UC-2 is what took a bit time, something would always go wrong during the last wave. To mitigate the issue with enemies piling up to fast, I got a sentry stuck on the top platform so it would denotate as soon as the first wave spawned. This made it drastically easier to kill them all before the final two waves. Even kept everyone alive so I'm happy with the end result.

Don't think I'll bother with the co-op levels. Prefer to get on with Sparks of Hope.