Minmaxing in D&D Next

I have a player at my table who is playing a death cleric. He is using a scythe as a polearm which we have agreed is doing a d10. He has some class feature that allows him to do an extra d8 three times a day for the entire combat. When he hits a target he is able to make a second swift attack. Finally, he has some kind of polearm feat that gives him threatening reach.

So while my other players are doing like a d6 per round this guy is often doing 2d10+2d8+2 which just seems completely broken.

My first question is am I missing something--I am fairly new to D&D Next and some of this character optimization may not be legal--can anyone validate that what this player is doing is actually possible RAW?

My second question is, does D&D Next support 'system mastery' in the same was a 3.0? Do 'advanced' players have significantly more powerful PC's simply because of rule mastery? 

I remember in 3.0 building encounters to just shut down certain PC's & prevent them from doing their 'tricks', I did not find I had to do that as much in 4.0 and I am curious if we are returning to those types of encounters?

Here is video of Thor's PC in action