I really don't give this one to Garruk if his opponent is Lili. She might have dealt with him by using the veil, but she controlled that fight from almost the start.
Unless she's dealing with someone else, his number is gonna get punched.

... and I just had the horrifying realization that anybody that dies first becomes hers...
Oh good god... The field is hers.

Garruk would likely come right after her, and Jace would try to stop him, likely getting mowed down in the process.

Assuming she can raise a planeswalker, anyway. But still, she's also likely to manipulate Jace into protecting her, which will buy her some time. Garruk would likely come right after her, and Jace would try to stop him, likely getting mowed down in the process. Chandra and Ajani fight it out while the other three are doing their thing, and that one boils down to whether or not Ajani can get in close. Lili will take out Garruk while he's finishing up Jace, leaving a relatively fresh Liliana vs the winner of Chandra and Ajani. Lili's got a good chance, I think, but I might give the slight edge to Chandra, depending on how angry she gets over the whole thing.

Of course, Nicol Bolas is the ultimate winner, since he probably set up the whole thing in the first place.

That's actually going against the prompt's basis since it states that they've been stripped of their relationships outside the arena.
Given that, to Garruk, the largest threat would probably be the duel axe wielding lion man.
He's a very physical kind of guy, so his immediate assessment will tends not towards their mystical capacity, but their physical aptitude. That actually would probably put Jace and Lili relatively low on his priority list.

Wait. Hunger Games with Teenwalkers?

Jace on the other hand has a full copy of Kallist's sword skills sitting around in his head plus his own training and experiences there, he's also the only one of the group with any experience or training in regards to planeswalker battles, having been trained by Tezzeret and fought Tezzy and others at times, like Baltrice and Ral Zerek. He's also got the advantage of blue-hax, and the potential threat of manipulating the other walkers into attacking each-other first to weaken them before he finishes off the remainder, and illusions to avoid being confronted directly that the others would have trouble dealing with or seeing through, and he's pretty exclusive in having stuff like counter-spells and un-summoning to deal with the biggest threats the others send at him, he's better suited than the rest towards battle melee against multiple opponents all fighting each other, due to his illusions and mind-magic

including a Lilliana without enough corpses or spirits around to control, as I get the impression that she is less efficient at summoning her own monsters as she is at dealing with nearby spirits and corpses and turning them into her minions. She'd probably beat Garruk if the battlefield was pre-strewn with corpses, regardless of the minor advantage he has from his close-combat personal skills.

As for Liliana, we don't really see much of her without some kind of thing powering her up under special circumstances, usually Bolas, but also the Veil. Although I'm not sure her multiple timeline zombie Bolases thing should count as cannon anyway, she's been protected from Jace's mind-reading and who knows what else and doesn't really fight much in Agents of Artifice.

The stuff she did on Innistrad seems to be the best example of her abilities on her own, but it's uncertain how much of what she did was taking advantage of local undead forces that were already out of control on that plane, and how much she did on her own, plus she wasn't really fighting peers in that setup, but bunches of non-planeswalker mages or simple warriors who are normally reliant on Avacyn for their powers, and are nerfed with her locked away. She also was hiding her true skill level while dealing with Jace in Agents, so again it's hard to tell there, and her tussle with Garruk before getting the veil was too brief to tell much, as well as her again using local corpses.

I think the only time we see her pulling up summons on her own is against Baltrice in Agents, and it was mostly spirit ones, and I got the impression that while Liliana was the clearly better one in that fight, Baltrice isn't exactly a stellar example of a planeswalker when it comes to power, something like a drake to Chandra's dragon in terms of power levels for a fire mage, or as a partly red mage using some magi-tech, an HTML user to Ral Zerek's computer science mastery.