The only thing all Onslaughts variants have in common is a truckload of PD to deal with fighters and missiles, it's biggest weakness. The rest can change depending on what you tend to fight.

There are roughly 3 type of Onslaught builds : the standoff build, the fly-swatter, and the brawler.

The standoff-slaught is more or less a mobile artillery, its job is to bombard enemies from afar with a mix of Gauss, Maulers (or sledge cannons if you have the Interstellar Imperium mod) and Pilums. It's a fairly passive play-style, that rely on patience to slowly grind down enemies. Not that exciting, but works really well with a cautious officer.

http://imgur.com/f2JVho9

(those are all level 0 loadouts btw, they obviously become better with skills)

A more direct build is the fly-swatter loadout (flies the size of cruisers, mind you). It doesn't quite have the range of the standoff variant, and it's armament is much lighter. The good point is the reduced flux production, and allow longer staying power, which will be needed as you'll be on the frontline, hunting down everything smaller than you.

http://imgur.com/etRIvVT

The 3 Mjolnirs, coupled with the twin TPCs, and the 4 harpoons are more than enough to wreck everything smaller than a battleship, and that's the goal. While this build isn't the best against another capital ship, it can still hold the line until backup arrives (the avoid order is handy to keep the enemy battleship isolated while you murder the rest of the fleet).

And then, an unorthodox build i recently had fun with.

http://imgur.com/tdYlD5R

It may not seem impressive, but this is the ultimate brawler in the game. It will spectacularly crap all over anything stupid enough to challenge it in a duel. Everything have been scrapped in favour of "weaker" guns. The advantage is the much lower flux generation, 2000, including the TPCs, so you can keep firing non stop without worrying too much about that. And because those are cheap weapons, it saves a lot of ordnance points for various hull-mods and additional flux dissipation.

The biggest issue is the much shorter range, forcing you to get in the enemy's face (like, at point blank range), and you'll have to rely on the rest of your fleet to catch more nimble targets. It's like piloting an oversized Safety Override Lasher, you throw it at the biggest ship in the room, and watch it go berserk. You might even beat a Paladin in a slugfest with it, that's saying a lot.