I'm probably not the best person to answer this, but I'll give it a shot.
Thufir can give the system argument, and I am going to try and give the other side. Freeform!
Well, Chrystal, I'm not giving him "the system option", because there's nothing you can only do in freeform and you can't do with a good system. The opposite isn't true, however.
In a freeform RP, it's usually best to have your players on the same team, because otherwise PvP can get messy. It's either that or you make it clear that any PvP must have the outcome agreed publicly in advance.
But in order to keep them entertained, plot twists are good. Throw in random NPCs, and have them do random things. One of the nice things about freeform RP is that your characters are constrained only by the setting and their imaginations.
So, to take the example you give of a handful of kobolds showing up: You (as GM) post that as the party round the corner, they come across a Kobold raiding party. You describe in as much detail as you like, the scenery, the weather, the kobolds themselves, what they look like what armour they are wearing, how big they are, the fact that the female one has clearly just been mounted by at least two of the males... whatever...
Then you wait. Your magic user posts next and describes the spell he casts at the Kobolds. Before you even get a chance to reply to that, your elf has fired six arrows at them and your paladin has rushed in with his sword drawn yelling "Death or glory!".
So you then post back describing the effects of each attack. You know your stuff well enough (I assume) to be able to simply make up what happens without rolling dice. It would just be realistic invention. So the freeze spell hits two of the enemy and they freeze, but the rest are sheltered behind the Kobold sorcerer's magic defence. The arrows are a different matter and one Kobold goes down with an arrow through the eye, another takes one in the sword arm, a third is hit by two, one in the chest and another in the belly, a forth enemy takes the last two arrows, one in his thigh and the other in his shield.
The paladin then charges in and his sword smashed the two frozen Kobolds. Then he decapitates the one with the arrow in the sword arm, before the female Kobold sorcerer casts a fireball at him...
And in the spirit of good freeform RP, you leave it for the Paladin's player to state what effect the fireball has.
And that here is the problem I thought the OP is asking about. BTW, I still think it's the case.
See, what you describe is a challenge to some people. It's a chalenge of stretching your creative muscles, throwing curveballs to the GM and other players to invent a fun response, and so on. I get that, of course
Yet the simple fact is that other players would simply get bored with it, not because you didn't describe well! To some people, if they're controlling the outcome, it's not seen as a challenge.
Their character might be hurt, but not as badly as to be taken out. That means there was no question how it'd end the whole time.
And that's the challenge some people want.
Are they bad players because of it? No, they're looking for a different kind of challenge. This makes them bad players no more than someone becomes a bad GM because of their preference towards using a heavily-plotted adventure, or their preference to non-scripted adventures...
So, I thought the OP is asking "how to give these people a challenging battle during freeform".
The short but true answer is "you can't give them anything they'd see as a challenge". You can give them a non-combat challenge, but to some people, the random kobolds simply aren't one.
Now, this might well change in games where the GM decides the outcome, based on the character's actions and his or her best judgement (hopefully including common sense and some knowledge of the subject). Or it might change in games that use some kind of random outcomes. But the majority of the games on E. are like what you described, the player decides what affects the character. And, as stated above, to some people that's incompatible with the word "challenge".
Hence my short answer before
Ah, now, that is a problem whatever kind of RP you use. In Furcadia they call them "Twinks", not sure if that word is used elsewhere.
Twinks and god-modders are the bane of the GMs existence.
But the way I would deal with such a player is, quite frankly, peer pressure.
See, I'd reserve peer pressure for cases where someone is spoiling other people's fun. Sometimes that means god-modders, indeed
Twinks, however, are easily dealt with, and you can actually transform them into nearly a boon for your game. Just use what they're doing into your game by having the NPCs react appropriately to it.
You have a bunch of great idea for freeform, Chrystal, and I have one more question for you; how would you deal with a party member that abuses healing powers? It wasn't a campaign, but I did do a 1x1 RP with someone once who'se response to every situation, be it trap, conflict, and so on was "I heal up and press on." Eventually that RP ended since I was too boring to challenge him, or so he said. I really didn't know what to do, outside of killing off his character.
(On a sidenote, I tried unhealable damage. He healed it.)
My GMing approach wouldn't change because of it. It's never about just winning a fight. So, whatever he does, he will be furthering the in-game events!
You've got epic healing powers? Great, that's a big rarity! Now, let me review the list of important NPCs and organisations in the setting...
You see, there's a group of people that would literally kill to get to you. And they want you to heal someone for them. They offer good payment, BTW.
But who the hell is this person, and why wasn't a lesser healer able to help them
Also, he fights on autoheal mode. Great, did he kill anyone? Ever heard of blood feuds? Healing your wounds can't help you if you're simply restrained and tied. That's not injury.
Yes, whatever he does, it's only going to help me as a GM. I've come to consider such players boons, not banes, as long as they don't pretend they can do everything better than everyone.
And if they do that, frankly, why are you playing with god-modders