The group in question has a light cleric, a celestial pact warlock, an arcane trickster, a paladin, and a fighter. They tend to fight with the cleric and warlock doing spell damage from range while the paladin and fighter tie up front lines with the rogue providing special 'striking' support and mobility. So let's begin.
Step 1: The 'Problems'
The problems in facing this group come in the following:
- Beefy, well armored front line that can deal solid damage to single targets
- Lots of AOE damage from the cleric
- Long range striking from the Warlock
- Significant amounts of healing between Warlock, Paladin, Second Wind (on the fighter)
- Cleric gives disadvantage on direct attacks against her
- Warlock tends to be heavily obscured and hitting her does damage to striker
There are however a couple of good points. Namely that the group does not always use optimal target selection and can tend to get a bit split up.
Step 2: The Villain's Resources
For the purpose of this, let's use a well funded criminal organization. I'm using them as they're not in the picture anymore in the game so it's not like this is giving away anything.
The villain has access to assassins for 'elite' units, and to large amounts of thugs and general enforcers. They're not particularly magically adept, but they're well funded.
Still, this gives them strength in that their elites are mobile, stealthy, and hard hitting to take out priority targets. Meanwhile their lower ranked people may be more canon fodder, but should also be able to tie up front liners for some time. They can also reach out with money to hire a third party ringer if need be.
Step 3: Solve The Problem
The Cleric: the problems provided by the cleric is long range AOE damage, while also giving penalties to attacks on her with warding flare. The solution is to use more elite units than lower rank units. Units that can stay apart from each other to minimize the effectiveness of large bursts of AOE damage. We also want multiple strikers dedicated to the cleric, one to burn the cleric's defenses while another goes in for the kill.
The Warlock: The warlock does long range striking support. They also have a tendency to be heavily obscured. They also provide a lot of healing. Damaging them also does damage back - particularly to melee strikers. This means you want ranged attackers. However, it is likely faster to just control vision around the warlock to mitigate the healing/long range striking until attackers can focus down or try to get through defenses.
The Fighter & The Paladin: The Fighter and the Paladin I combined as they both approach the same way. They move into melee and they unload damage. Both have ranged options as well, but prefer getting into melee. We don't want them in melee, so we'll need to use positioning and mobile units to play keep away. When this fails, being able to disengage or deny sight will be vital. Ideally at least one target is down before these two get in range, but that isn't something you can count on.
The Rogue: the rogue presents large problems for this group. He is mobile, can go invisible, strikes hard, and uses all their strengths. To beat him would require trickery.
The big catch here is any attempt to target the PCs to take them out would be a massive investment. One liable to break the group should it fail, and that is a high risk because we're dependent on an intricate plan and parameters to work. Not something to take lightly.
this plan is not super unique to this group of PCs. Taking out the mage (AOE damage dealer), mitigate healing, and avoid the front lline is standard tactics. PCs use this all the time. However, what makes adventuring groups so deadly as groups is their well rounded nature. Which is not something you always take down.
The real way I'd see criminals and assassins taking down this group is not in a group fight, but one at a time. Ideally in a city or other area with corridors and objects to use to protect sight lines and increase the risk of collateral damage.
But that's not particularly fun for the PCs now is it? Which is the other part of this that we always need to remember. It is a game, it should be fun. So consider your group before you go fully "but this is how they'd try to win." Some groups will love it. Others? Not so much.