Combining a Lilim with an elemental attack eidolon provides a damage boost, but if a character dies you're still screwed. Either you sub in a primary element character and risk losing your buff, or you sub in a 4th element character and risk only two characters getting the buff. Either way, you're gambling and you're going to lose about half the time. The small damage increase isn't worth the additional risk in difficult content. You might as well run the 3/5 matching element team with usual eidolons.