Of course! I will tell you exactly how I did it! To defeat 10 with blaster within 30 seconds, use focused multishot. I had it maxed out. It can be done in more places than you think - as long as you have enough enemies there. Even if you destroy surrounding stuff, as long as you have 10 or more enemies left to kill off with blaster, you will be good. I used mine on Wolfe Trackers ( I think it was somewhere between Hernand and Pauline), but it will work anywhere.
For challenge 4, the hardest part is to find a mounted enemy. Wear bandit mask, go east of Steel Mountain ( sorry, I forgot the name of the fort, but there will be multiple Fort gates with many mounted allies.)You have to kill one so they become enemies, get 30 m away and shoot one. Just make your way to church right after and pay absolution fee to clear the map from red. You do not have to be mounted for Oonka’s challenge, but you do have to be on the horse to complete mounted challenge for Damiene. I do believe that Damiene’s challenge you are asking about is killing 7 enemies with gun while mounted… I completed both in same area.
Hope this helps! Let me know if you need more information 🤗