I just think it’s about time we had lvl 3 for the scb. It can increase in difficulty so it has difficulty as follows (using UC as standard) :-
Expert-3: UC gatekeeper 500
Master-3: UC gatekeeper 800
Extreme-3: UC gatekeeper 1000
Reward and exp wise :-
Exert lvl 3 : 2 SE pots.
Master lvl 3 : 1 (or 2) Leg pots
Extreme lvl 3 : 1 (or 2) rare gems.
In terms of exp :-
Expert : 50k
Master : 75k
Extreme : 100k
In my opinion lvl 2 is already a good challenge. There are various events which offer tougher challenges and then PvP is always going to be tricky because we face other people.
By turning up the buff as high as 500, 800 or 1000 UC that’s no longer encouraging people to create good teams and play cleverly, it’s simply pushing them to use some cheap strategy or use a good UC team. UC as an event is fine for this because the whole event is about beating insanely buffed monsters but beating 20 or 24 monster teams that are well made is a tough feat as it is.
I personally feel that players who can make good UC teams already have a nice advantage for the number of gems they can get from UC (50+ gems for players who can go lvl 700+). Giving them a legendary potion and a rare gem extra each week over players who cannot do this is a huge advantage. I think this would most likely make a lot of players angry, only a few could do these challenges.
If you want more difficulty added to SCB I would suggest a “bonus battle” being added which is “extreme lvl 2 - extended”. That would have the same buff level as lvl 2 but have an additional 6(?) monsters added to the enemy team which are all from the featured list. They’d be spaced out through the enemy line-up at places where they don’t disrupt the combos. The reward would be one gem. I reckon that would be a reasonable extra challenge for those who find themselves doing SCB first time every week.
Mate not everyone have the sleep lock monsters to beat UC level 1000. Additionally think about the achievement gems. They would be so much harder to get…
What if I told you the best strategy for UC isn’t sleep lock or stun? 
Are you intrigued? Ask my boy @SoL_Navelstan
Then I wouldn’t believe you. Some form of crowd control needs to happen to go that far.