So you are playing in PFS, and you run up into something with damage resistance 10/cold iron or silver. Normally, you would have to either be a lesser summoner and use the feats to give the eidolon the equivalency or wait till level 7 to get a bodywrap of mighty strikes +3 that only applies to 2 attacks anyway. There is, however, another way. Elemental damage bypasses normal damage reduction and an eidolon can wear rings. Equip the eidolon with two rings of retribution (or if you wear the monkey palm amulet 3) and have him charge into battle. Since the rings can be detonated as an immediate action, the eidolon will be able to use them as it is being destroyed by attacks of opportunity. Boom, 20d6 worth of fire damage. Worried about the eidolon dying? Hey, it can be summoned back in 24 hours. Coincidentally, the rings can be reused in 24 hours too. Bombs away!