I’ve been a DM for about 3 years, and have predominantly run one-shots and short campaigns in DnD5e and PF2e. I have a player who persistently builds primary caster based characters, but then won’t do anything in combat but “I stab it with my dagger.” They rarely use cantrips, and basically won’t cast a leveled spell unless I suggest it immediately before their turn. They seem to enjoy playing despite the fact that they’re far too squishy to be a front-line melee character and don’t utilize most of their class features. I’ve talked with them explicitly about how their play style seems to be discordant with the kind of play they want to do, and that maybe next time they should try a paladin/champion or a fighter/rougue subclass with some minor casting. They agreed at the time that sounded like a good idea, but low and behold showed up to the next one-shot with a primary caster, and over 3 hours of play and 3 combats never cast a single spell, including a cantrip.
I enjoy playing with this persons as a whole. They are engaged in the fiction, and are particularly engaged during exploration activities. They tell me they also find combat quite fun, and they are requesting I run a mega dungeon in the near future.
As a general rule, I like to let people play how they have the most fun, but issues have arisen with this play style. Namely, all of my TPKs have been associated with this player charging a squishy character directly up to a significantly stronger villain and continuing to stab it with a dagger until they went down, significantly hindering the party in the action economy and resulting in a TPK. I feel I have to intentionally weaken all of my encounters to keep the party feasible in the face of such mechanically poor combat choices.
What else can I do to help drive this individual towards melee builds, and/or help encourage them to change their play style to better suite the caster classes they choose?
I think you, as the DM, should privately poll your players to see if the choices of this individual are causing distress or reduced enjoyment for the other players. If so, you should consider one or all of the following:
Choose monsters for the next campaign that are physically immune / resistant to piercing damage. For example, every monster has a hard shell like a turtle or something and unless attacking with advantage or some other mechanic, the piercing attack will just glance off.
Create a back-story for the campaign that makes all monsters generally immune to most damage. For example, have them campaign to a plane of existence where the monsters need to be made vulnerable through finding certain artifacts that will essentially severely weaken or kill the monsters for them once the artifacts are found and engaged together.
Create a back story to the campaign that promotes choices in-line with your class and alignment. For example, make the monsters 20% more difficult, but allow the players automatic advantage if they choose to play / engage the monster in accordance with their class and alignment or automatic disadvantage if not…
Create a back story to the campaign that captures or incapacitates the problematic hero, rendering him useless unless he can find an artifact or a way out of some dungeon, maze, or cell, but the key is to use some specific element of a cantrip or spell and have clues that they can find to help allude to the issue. All the while, the other party members are progressing in a separate parallel quest.
In each scenario, the players will be forced to change and embrace different tactics or be dead weight. Its a hurdle / handicap everyone will need to overcome, but if they realize early that early on they can find a way to use it to their advantage…
If none of this works, brutally kill the player’s character or kick the player out, especially if the other players have had enough.