@ Thaago
It works even against Ordos if Onslaught has other ships supporting and taking heat off of it. One time, I took a low-tech fleet plus one or two Dooms, sent it against an Ordos with at least two Radiants, and the only casualties were two Enforcers. With smaller ships and Moras keeping pressure off of Onslaughts, it was a best case scenario for Onslaughts, and they tore up Remnants.
However, as a lone flagship or one of two tanking for a Astral and Drover carrier fleet, Onslaught is often a dead duck. It has trouble when mobbed.
You are correct that Devastators are on the sides, and another heavy needler in the center. They are anti-small ship, anything destroyer-sized or smaller, since the ships that will likely flank Onslaught are small fry, which Hellbore will have trouble hitting. I tried less heavy needlers and one storm needler, but the storm needler lacks range. The main advantage of heavy needlers is range (more than storm needler), but sometimes, having too many fire at once causes too much of a flux spike. Advantage of storm needler is more steady flux use and less OP than three heavy needlers.
Relying on TPCs only for anti-armor is a bit of a pain.