If all you want are the solo rewards, duel credits or t2 health potions give all the points in 1750 units.
However, if you want the alliance rewards as well, probably better to trade t4 alliance health potions, since those will be the most efficient in points in that event, and finishing there will complete the solo as well. That'll be done in about 3000 units. As a note, though, there are 550 units in the milestone rewards for alliance gifting and 20 in the solo milestones, so that'd be 2430 net units, assuming the rest of your alliance contributes.
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However, if you want the alliance rewards as well, probably better to trade t4 alliance health potions, since those will be the most efficient in points in that event, and finishing there will complete the solo as well. That'll be done in about 3000 units. As a note, though, there are 550 units in the milestone rewards for alliance gifting and 20 in the solo milestones, so that'd be 2430 net units, assuming the rest of your alliance contributes.