Rheios
Have him stab the mayor who’s evil because he’s greedy and selfish and borderline abusive in trade-deals with neighboring regions but is otherwise beloved (and has rewards heaped on him) because he’s so good at actually keeping order in the town and keeping their goodwill (although probably at least a little bit through some passive-aggressive blackmail). That’s always fun.
Yes and no.
If he’d gotten powers from the divine oath-giver he’d be a Warlock or Cleric, dependent upon the nature of their relationship and the being’s powers.
If he got the powers himself from his absolute rigid dedication to his oath, then he’d be a 5e Paladin (I prefer “Dedicant” or “Crusader” for which Paladin should be a specific Oath but that’s a different conversation).
Otherwise in older editions he’d probably just be a devout warrior.
For those older editions he’d only be a Paladin if the oath he held to was far more specific and arguably he and several of the other hobbits were a bit too quick and dirty for. Particularly during the era of Racial restrictions to classes which didn’t allow halfling Paladins. (Assuming halflings to hobbits is 1:1 in all settings, which is far less consistent over time.)
For how a generous DM might work around that in older editions sometimes, I’d look to BG2’s Mazzy Fentan: https://baldursgate.fandom.com/wiki/Mazzy_Fentan
…I miss when Time Stop let you cast attack spells…
Have you considered an anti-magic field area of considerable size? That’s hardly a good continual response but it may allow a more challenging encounter, especially if the field can be briefly turned off by characters expending spell slots into a single dangerous spot while being attacked. Since it prevents Divine smites and the bonuses. (Someone could 5e-argue that probably, since its pretty nonsensical with class abilities at times but I’d just overrule it.) You can even use it with Mindflayers or other psionic aberrations, since arguably their abilities can still work if you rule they can. (Psionics don’t necessarily equal magic unless you set some other precedent.)
Other concepts are to add hazards and chaos causers to battlefields. Floating clouds of glittering fog that reflect spells to random other targets, negative-energy/undead quicksand bogs that leech endurance instead of doing damage (and whose saves are to avoid getting stuck not to avoid the drain), unsteady floors to drop out and separate parts of the party, or lair effects that randomly teleport characters back to earlier areas of a dungeon, forcing them to run back through to get back into the fight (don’t overuse that one but if there are traps earlier its a great way to force them to ignore traps in their rush).
Also just tasks to do while being attacked. The paladins + cleric can defend well together but force them to be separated into different regions doing a task and you up the tension (even if you don’t up the danger).
You can also try bringing forward older monsters that undercut benefits/items. Black Puddings, Rust Monsters (Or their papa Annihilators), or port over other monsters like Magerippers or Spellweavers from 3.5.
Traps can also be good, since they may be taking 1/4 damage on a save, but if the traps also cause inconveniences or force them through alternate, and slowly more damaging paths (like crawling through a stone brambled tunnel instead of taking the other tunnel because it caved in and almost crushed them), they can still build up and be valuable.
Also if they’re incredibly proficient in combat, the enemies probably know that by this level. So you might have to start attacking them legally or socially, depending on the situation. Or just start having foes avoid them. Make them burn resources to set up engagements where foes can’t run or attack them when they’re on their back foot relaxing with assassins in the bathrooms, poisons in the bar bread, false accusers of horrible crimes on the streets and in court. (Depending on who their foes are of course.) You do that so that their character get paranoid. Start trading things like spell slots, the benefits of sleep, or close allies to try and defend themselves, so that they’re weakened before they even touch something like a dungeon. If they party is just too invincible in combat as it is, don’t just attack them there, let them know that existence is sortof a threat. (And as before, how much you use it is important an should be informed by session 0. You want them tense and excited, not miserable, and “the world as the DM’s weapon” isn’t necessarily the right way to approach it but its a nice tool to have in the chest. Hammer finds nail and all that.)
Simple rules that can describe almost every situation are also rules that over-generalize characters to the detriment of options (everyone’s noticing the same things, instead of perception allowing more observant characters to do what they could do), over-include the player’s capabilities in place of the character’s. (Players conversational skills failing to match with those of the character they intend to play), overly abstract what they describe (a monster’s “power” or a character’s actual abilities meaning something in adjudication but nothing consistent/concrete enough in-world), or demand a DM adjudicate without reinforcement or restriction (In the absence of rules every corner case ruling risks the danger of turning the table into a debate between PCs and the DM, inviting rapid ends and either producing embittered DMs or embittered players* - especially under the “pack it up” approach the video suggests - and helping to increase combative tables in the future.)
The games that OSR takes inspiration from did a lot right in their mortal power-level, reasonable growth, real risk of danger, and humanistic tones but if you’re trying to sell me that the growth of rules that followed aren’t a direct result of weaknesses in those games? I don’t think we’ll agree.
*The “Dorkness Rising” problem, for a slightly more light-hearted allusion.
Killing him altogether seems pretty epic level, like level 25+, given that he’s a deity. (But your DM could be ballsier than me, lol. Killing an aspect of him to weaken him for a bit seems more my speed.)
Alternatively you could try shifting goblin worship in localized communities to another deity. Maybe someone like Kikanuti (since I imagine getting them to worship someone like Tymora immediately might be too much of a jump?) or some other goblin. (Were Konsi to be more arrogant, I’d suggest her. =P) Kill him slowly, death by 1000 cuts of lost faith style.
For D&D I can also think of Lolth, Tasha//Iggwilv (depending on your interpretation of that mixed bag of lore threads), Ravel Puzzlewell (a hag so powerful or madly knowledgeable she has aspects across the planes from the Planescape setting), and Loviatar the Mistress/Goddess of Pain.
There’s also existent queens like Queen Mary(“Blood Mary”) that oversaw religious purges that you could take inspiration from. (Zealots are rarely reasonable enough to be bargained with.)
You could also just take a historically male example and invert the sex and not make a deal about it. See if your players even notice.
It is worth noting that the -Con score was a 3.X house rule but Pathfinder 1e raw. It was just -10 otherwise, which could get pretty punishing if you were dropped by bad luck.
5e’s up-and-down approach to unconsciousness isn’t really an ideal resolution, although making them gain levels of fatigue almost makes it functional.