sindarin translation of "Help him!" ?

The best I've been able to do -- and it's probably awful! -- to translate the imperative sentence, "Help him!" would be "E-grest". I have no idea what the exclamation point equivalent is.

Any better ideas?

I would suggest natho den! (natho den ! or natho den ! in Tengwar). It’s the imperative of natha- (which is attested later than gresta-) + the object form of te ‘he/she’, which I think is better supported than e.

Thanks!