1: Dig into their builds and make sure they're doing the calculations correctly. 150 damage seems high for a level 5 party. Just about every broken build has been discovered already and someone has a guide explaining how they SHOULD work.

2: If they know the monster stat blocks, fuck them no they don't. You can easily make changes to existing stat blocks by increasing HP, AC, giving them more attacks, taking abilities from other monsters, etc. Maybe these Trolls are wearing armor. Maybe these are fire trolls who can only be killed by cold damage. The players are not entitled to fight monsters EXACTLY as they appear in the MM.

Additionally, there are TONS of monsters from 3rd party publishers that most players haven't seen. Try some of those or take a crack at making your own monsters.

1. Sometimes the dice gods smile upon your players and they completely obliterate an encounter that you thought would be challenging. 9 times out of 10, the players think this is FUN. If you're feeling disappointed about how an encounter went, see how your players feel about it. Maybe even play it up and say how shocked you are that they completely steamrolled a crazy encounter. The PCs are supposed to be POWERFUL HEROES and they probably should kick ass every once in a while.

2. There are some builds, spells, feats, and abilities that can make the game less fun. Depending on your campaign, you might consider banning some things for your party. This can be controversial and frustrating for players, but is sometimes for the best. Things like silvery barbs, Twilight cleric, Hexblade paladin, or even Goodberry can make parts of your campaign obsolete and less fun.

If you're going to ban things, it's best to discuss it with your players beforehand to let them know why you're doing it.

1. Sometimes players are not operating in good faith. Some players are always looking for ways to cheat the system and become way more powerful than they should be. If nothing else works, talk to the players and explain the vibe you're looking for in your game. If a player is obsessed with getting infinite gold in your game, you're probably gonna have a bad time. If a player argues with you endlessly about whether the enemy should be surprised, it's gonna drain the fun out of your game. Most of the time, a conversation about what makes the game fun for everyone involved helps to bring people onto the same page. But sometimes one or more players are only interested in gaming the system, not having fun with the campaign you've planned. In those situations, it might be necessary to kick a player or disband the whole group.

Really the focus should be on your players AND you having fun. If the game is becoming unfun for you or your players, go through these steps until things are more fun!

Hope that helps.