For turn 1, Nagi has a modifier of 17.5x, AFry of 20.5x and Elena of 16x on their AR skills. Nagi has the 6x chain cap, but she can't triplecast. She also has to deal with the wrong element. Assuming that in all cases you have an external 120% imperil, the effective multipliers are: 17.5*2*6*(1+(1.2+0.5-0.8)/2)=304.5 for Nagi, 20.5*3*4*(1+1.2)=541.2 for AFry and 16*3*4*(1+1.2)=422.4 for Elena, so for turn 1 you should keep AFry and Elena.

If you're planning a turn 2 burst, Nagi can use Dance of Darkness, that increases the modifier of the AR skill in 20x and imperils dark by 100%. AFry can't increase the mods of her AR skill, and Elena needs two turns with her unlocks. So the effective multiplier of Nagi is 37.5*2*6*(1+(1.2+0.5-0.8+1.0)/2)=877.5, far better that any of the other options.

tl;dr: for turn 1 clears use AFry+Elena, for turn 2 clears use Nagi+AFry.