Having Clark distract him (which would be painfully easy for someone as fast as he is) while Diana ties him up and makes him renounce the Gem's power, followed up by a one-punch from either of them is the teams best bet.
Alternatively, decapitating him with the Tiara may be a secondary plan of action.
Both of these may or may not work. Magic is kinda funky like that. If they're restricted to pure melee battle, Juggernaut would eventually win, but it'd take ages considering that he'd never lay a finger on either of the duo, assuming they didn't want him to. Further more, each of them have nigh-infinite stamina. So, a pure melee confrontation is certainly Juggernaut's win....after a ridiculously long amount of time.
If Diana's weapons work effectively, the duo easily win.
If they do not, then, well, it's back to a pure melee confrontation - see above.