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So I am running a campaign as a DM, and one session my players were in a dungeon and spotted a Chuul in a water-filled area. The Chuul stayed deep in the pool, so the characters decided to drain the pool over time using a series of spells to over time expose the monster (Control Water to turn some of it to ice, Mage Hand to bring it away). They also kept their distance. I felt a little bit frustrated, because if I ever attacked them with the Chuul, they would run away while blasting spells and arrows at it, and the Chuul wouldn't stand a chance.

I kind of got lucky in that situation, as the players strayed too close, but it got me wondering, as the players may have gotten out of the boss fight I had intended.

How can I engage players with a slow melee monster, when the players can keep a distance from the monster and use ranged attacks?

Please do not make your answers case specific (e.g. an answer that only applies in a dungeon).

Add allies that move, fly, or have ranged attacks

Your players employed good tactics to deal with a particular challenge. Using ranged attacks is such a great advantage in combat that in real life, ranged weapons keep on getting better over the years.

Since you want to challenge your players, you as the DM need to set up a situation that forces them to make another tactical choice: focus fire on the slow enemy, or deal with that quick / nimble enemy, or try to do both. Each choice has a consequence.

Example: the group I DM for were battling a horde of zombies, and two zombie ogres, but there was also that nasty leader of this group, a wight, who was shooting at them with a long bow. Wights have some rather nasty side effects on successful hits in melee, but this one was trying to put the spell casters down first to then get into melee and make another zombie out of a PC. The party had to figure out how to break up the synergy, since the intelligent wight kept targeting the spell casters with his bow shots.

As it turned out, the party's cleric turn undead attempt was so effective that the bulk of the zombies ran off, leaving the party with a less difficult problem to solve. If fewer undead had missed their wisdom save ...

Back to your problem: single monsters are susceptible to "focus fire" from the entire party. Give them an ally, or a side kick, so that the party has more than one problem to solve.

The players may have gotten out of the boss fight I had intended

Excellent - I love it when players outsmart me, giving players the chance to feel clever and empowered is what being a DM is all about.

Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win.

Sun Tzu

If you've got a group that are using creative combat solutions you are going to need to do the same thing to challenge them.

In general, a slow melee monster needs to contain or immobilise faster opponents. So you want to think about setups that trap or corner the players with the monster. How that is set up will be completely dependent on the situation.

In the example you give, it sounds like draining the water would take a very long time using their method. What else is happening during this time? What else is in the area that might wander up behind them, sandwiching them between the Chuul and the wandering enemies? What was the Chuul doing all that time, aren't they fond of burrowing and ambushing their prey? Would it have a network of tunnels throughout the the area that it could use to pop up behind the players, trapping them between it and the water? A pit that it had burrowed out that it could collapse under the players?

The party is sitting around playing dice while the magic users painstakingly haul blocks of ice out of the water. The ground under them begins to vibrate and suddenly collapses, dropping them into a deep pit (roll to dive to safety) with an angry Chuul. Also, the pit is rapidly filling with water pouring through the tunnel it dug from the bottom of the lake. Roll initiative :P