Cobalion's Base 192 Attack isn't great by PvE standards. An even worse movepool in Metal Claw, Zen Headbutt, Close Combat, Iron Head, and Stone Edge means that Cobalion should never see the light of day in PvE. Metal Claw/Iron Head isn't a bad Steel-type moveset, but Cobalion's Base 192 Attack means it falls below Metagross, Dialga, Scizor, Durant, and even Aggron's level of performance.
That said, a Tier 5 Raid is a Tier 5 Raid, and it's still worth doing Cobalion in small groups to maximize Rare Candy gains.
Outside of Sun, Cobalion is a fairly easy trio.
Minimum viable levels assume Best Friends, Sunny Weather, no Dodging.
Italics indicate a Legacy move.
Level 40 Counter/Blast Burn Blaziken falls a little short of being duo-viable by itself; if you have enough Chandelures and Moltres carrying, a few Blast Burn Blazikens can be safely slotted in.
Level 40 Fire Spin/Overheat Moltres falls a little short of being duo-viable; again, if you have enough Chandelures carrying, a few Moltres can be slotted in against Close Combat.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
Compared to the Sunny duo, trioing Cobalion is much easier, and many more attackers are trio-viable. Against Iron Head, Fire-types do very well due to their resistance to Steel.
Overall, Close Combat is slightly harder than Iron Head to go against. However, Close Combat is also easier to dodge, and this fight's DPS check is lenient enough that you can afford to dodge a bit to save on Revives without jeopardizing your run.
Stone Edge hits hard, is difficult to dodge, and hits Fire-types for massive damage, especially Moltres and Charizard. Fighting-types, due to their resistance to Rock, perform a bit better against Stone Edge than Fire-types.