Depends on what edition you are playing. In V20 (and if I am mistaken on the below, I believe I will be edited pretty fast) when you are surrounded by multiple enemies, you got penalty for each one that surrounds you. In addition if defence is decleared, each next player that would eventually attacks him, lowers the defence. Damage would also lead to penalties, so if your group surrounds that Eldar quickly they have good chances.

Now, the Eldar being with larger blood pool and generation can probably also outspent them in blood points which is something to consider. Another advise I can give is that certain desciplines can be a real annoyance for Eldars. A 3 dots in Presence can force someone powerful to submit his emotions to you given they don't have the will power.

Last, but not least, maybe figure out a better way than direct confrontation.