Notes

Existing challenges that I have criticisms of:
Seed for these: 596
Inflict toxic (one attack)
Kill someone via greatbow knockdown fall/Knock someone off a ladder/Kill someone via gravity (The first and second are hard to get without the third.
If they are all in a line, it makes the line very easy.)
Win 3 invasions in a row in X (Pretty good except that these sometimes have very different average levels in one line.)

Parry someone with the Farron Greatsword dagger (Very good because it's difficult, but it's also a boss weapon, restricting usage.)
Kill someone using Pestilent Mercury (AFK darkmoons?)

-I then found the challenge list.-

Kill someone with Sacred Flame without the exploit (Exploit removed.)
Kill a Red as a Darkmoon/Sentinel (Depends on autosummoning.)
Kill an opponent with a single attack (Very heavily depends on enemy build. Probably requires low level glass cannon.)
Demon Fists/other boss weapon challenges (Requires boss weapon)

Suggestions for Challenges:

Win an invasion in a seeded world without landing a hit on the host. (Only requires the world to be seeded)
Finish an opponent off with a status effect. (Poison resins, dung pies)