If your luck runs like mine does, I can guarantee that you will get a Valraven in the first room after you purchase tier 1.
I think I was able to complete all tasks in a weekly event one time, without using any gems. I’ve since just been purchasing two by default, just to speed up the process.
I normally spend the 30 gems at start just so every troops starts enchanted. Fast teams even faster.
Yeah, the guaranteed finish has always been purchasing tier I. At least it isn’t like class events where you have to pay regular event prices and buy tier 5 to finish for one orb total. It’s just 30 gems.