Welcome to sandbagging. Some of the more developed players drop trophies intentionally to manipulate the match making for quick and easy wins. Is it unfair to the new guys? Yes. What can you do to limit your encounters with these cowardly keepers? Their goal is quick easy low cost wins to collect stars for the tournaments, so bolster your dark temple so they have a higher cost in mana when raiding. Avoid single tile walls, and upgrade your reinforced walls. Spread your rooms out so they are less likely to be able to hit them all in the time frame without sending out alot of minions.
What can you do to revenge? I like to refer to it as loot bagging, (i've been promoting the term for a few years but yes I still have hope that it will catch on) drop a few biles or skellies on their warehouse and treasury, take what you can get. Yes you will lose, but at the same time you win. You build up xp on your bombwall and whatever other spells you feel like practice. You get whatever loot you can grab for the cost of next to nothing. You'll only lose one or two trophies which are easily made up. Cluck off their immortal if you can, theyll have to wait for revive or pay gems to retrieve. And most importantly, they will gain trophies they don't want.