I have a group that is very mobile and I want to start challenging them more/give the enemy caster a turn or two to get cooking. What are some good enemy frontliners? I realize I could just have the frontliners they run past just do the same thing and go after the casters, but that feels a bit chaotics and I like me a nice setpiece now and then.
Really depends on what level you are playing, I like Graveknights, pair them with a lich or a vampire, and a bunch skeletons and zombies fodder and you have a great boss battle.
I like throwing some ruffians from the GM guide at the players. Snagging strike is really strong. They fit in well with any group of bandits, robbers, criminal organization of choice or as hired thugs for a noble.
Ropers are very good at denying movement. For the right party, even just one roper can be a challenge.
One of my favorite encounters when I ran 5e was using a roper on the ceiling while the party fought a couple ghouls. The bard and wizard were very surprised 👹