Hi sunt eidem quos saepenumero Helvetii non solum in suis sed etiam in illorum finibus superaverunt
They are those whom the Helvetians time and again defeated not only in their own territories but also the territories of others. Not sure what do do with eidem.
Almost there. You should know that eidem is from idem, eadem, idem. Also illorum is not other (that would be aliorum) but it refers to the subject of the principal. Give it another try.
I will try again: They are the same people whom the Helvetians time and again defeated not only in their own territories but also their(the German) territories.
Hopefully this is better Bedwere: They are the same whom the Helvetians time and again defeated not only in their own territories but also their(the German) territories.
In this sentence it is nominative plural. Writing down the declension table didn’t help so I consulted an online source and found my error. No wonder I had such trouble with a really simple sentence.