@BonhommeCarnaval: Right, except that the Onslaught would likely have Maulers or HVDs mounted as well, and potentially even Gauss Cannons that outrange everything (true, you can mount that on the Conquest as well).
What works to the Onslaught's advantage is that it does not need to enable its shields at all. I usually go with 2 Flak + 3 Dual Flak, and this, along with the thick armor, means that the relatively poor front-facing shield is unimportant.
Hypothetically, the Conquest would be focusing on dancing just out of range of the TPCs and the Onslaught's 3x Large Ballistics, while trying to loop around to the Onslaught's rear. At such a large radius, however, it's nigh impossible to dance around faster than the Onslaught can turn. Moving closer isn't an option, however, as the Onslaught can tank some hits without its Large Ballistics being disabled (as they're mounted far from the edge), whereas the Conquest will either have several of its weapons disabled or take a large flux hit from its poor shield efficiency.
What really ruins the Conquest's day, however, is the flux-free constant stream of Annihilator rockets that the Onslaught can unleash. Unless you mount at least 2x (Dual) Flaks, neither the energy PD nor the shielding will be able to keep up with it. The Onslaught can, on the other hand, shrug off any missiles from the Conquest due to the Flak wall.
That said though, both ships have their uses, and I do like the Conquest as well. I would personally argue that Onslaught is a better 1-on-1 brawler, while the Conquest performs better in dominating a field versus cruisers and destroyers. It's why I'm using a Conquest in campaign ATM.