I have to say most are pretty doable honestly. Specially for Grail Knight.
Outcast Engineer a bit less so, since some are just WAY too tedious.
But you can just skip them after all, bit of a waste of the money but still.
So far the only weird ones on Outcast Engineer which are basically just game breaking if you ask me are TARGETING ARRAY, ONG TUK DWE FUT SAK and INTERCEPTING BARRAGE.
What I mean by that is that you have to start setting up a specific scenario just to get these. Cause in a normal game you would NEVER, EVER get these. All the others I could see you hitting simply by accident at some points, specially if you know they are there (like getting halescourge with the cog hammer). Just play with the cog hammer, and you will end up with it at some point in time.
Problem with Targeting Array, even now, is that you will use it for other stuff in the meantime. So you have to go specifically out of your way not using your ranged weapon because you need subsequent elite headshots. That is not even getting into the terrible hitboxes making a clear headshot sometimes go through enemies. In general, just a bad challenge that isn’t worth the time, and I think the only reason it is there is because they wanted another challenge but couldn’t come up with something good, as simply headshots with the masterwork pistol is already a different challenge.
The problem with ONG TUK DWE FUT SAK is, why would I waste THAT MANY bombs on a boss EVER!? So again, not happening by chance (besides carrying a maximum of 3, you would have to play with bots that will feed you bombs, or ask your team for the challenge specifically). Not as bad as targeting array, but still in the game breaking categorie as you need to set it up specifically and will never ever happen by simple chance.
The problem with INTERCEPTING BARRAGE is, why? Why would you throw a bomb at a leaping ogre? Chance of missing is high unless he comes at you, and if he comes at you you better be blocking and/or dodging. Again, not too hard to set up, but again needs to be set up cause it will unlikely happen by accident. More likely than the above ones to happen by sheer luck/accident though So this one isn’t THAT bad, but…
Now you could say Grail Knight having to interrupt deathrattler may never occur, but I feel that there is a decent chance that it could happen by accident, and on top of that it doesn’t have to be set up at least as a scenario. Just do the map often enough and keep in mind you need to interrupt him and you will get it at some point.
Anyway that is how I feel about the challenges. Grail Knight was well done and I agree that OE is a lot less thought out properly, but Most of them are very doable, even the alt fire one (just get really close to the monster).