I still think ranged character might be able to beat it relatively easily. Especially with a FE:Giants ranger/horizon walker, or somesuch...
Lessee, something like Greater focus archer fighter, probably move+multishot, as there's the whole hitting issue. STR 14 could be adequate, presuming Bull's STR or a +4 item. Bow would be +2, Dex... 18 naked, presuming elf, so 22 boosted. To-hit of +20 vs. AC 27, doable. Damage isn't that good at 1d8+8, presuming WS and a +2 mighty (4) bow.
If we simply clone five of them, they deal 100ish points of damage per round, and take the giant out in 2-3 rounds under favorable conditions. The giant will likely kill one, maybe two of them, but I think that's about it since it's denied the chance to attack the whole group at once. Without too much investment in AC, it should still be in the 22-27 range, so the to-hit advantage will start to show.
Now, of course, if the giant can reach the group in melee, they're in a bit of trouble. Higher ground or flight would certainly be favorable. Still, they do require access to magical gear appropriate for their level to have a chance.
These are with a cost of 5 feats spent on bows, so they're pretty dedicated archers, really.
Looking at the title, and the storm giant, it seems what is needed is an AC of 10+3x level (or equivalent defense), an attack at least 2x your level, ( between 2x - 3x level range ), dealing at least your level in damage, and ability to withstand about twice your level in damage, for four-five rounds.