Here's the real problem: There is no cost associated with high level players dropping trophies and having a "feeding frenzy" on players who are unable to protect themselves by staying online for hours at a time. Previously the barrier to this behaviour was the time, and to a lesser extent gold, required to next through villages that did not provide enough loot to warrant attacking.
My solution:
1. Leave matchmaking as it is. More time actually raiding and less time nexting is a good thing overall for the game. Raiding is more fun than nexting.
2. Introduce more risk into the equation for players who choose to drop trophies and prey on more casual players. I have some suggestions.
a) Increase the cap on loot available for players who are attacking above their own TH level.
b) Provide a loot bonus for players who are attacking above their own TH level. or;
c) Both.
This would give players who cannot stay online for hours at a time an incentive to train stronger armies and give them the ability to profit and progress.
It also introduces an element of risk for the players who drop down for a "feeding frenzy". They will still make loot hand over fist, but they are exposing themselves to players who will be incentivized to hit them with stronger armies.
The people who are able and choose to play in extended sessions will still come out far ahead of casual players, and more casual players will be able to mitigate their losses through increased gains.
It's not a good thing when the line between casual and active in a mobile game is drawn at being able to keep your phone from locking for hours at a time.
In my opinion, if the players who are so keen to destroy more casual players find this to be an unacceptable solution it is a strong indicator that they are not interested in a more competitive game. Just admit that you want the game to be easier, but only for you.