Onslaughts have poor turning and a vulnerable rear (giggity). Good strategy would be tearing away at the escorts where possible, either don't deploy the Paragon until they're dealt with or put an officer with elite flux modulation in it and just use it as a distraction. Once that's done, try to split the Onslaughts and remaining escorts from one another and send all the smaller, faster ships to swarm one while the Paragon pins down the other.
Honestly, sometimes I prefer faster cruisers for the purpose of punishing any frigates/destroyers that decide to leave their battle line. A good number of strike weapons means you can cripple or destroy them fast then back out before you start getting smacked, and heavily damaged ships often retreat.
Hyperions can take down Onslaughts too if you control them and just keep behind.
EDIT: Everything was perfect. I had a wand that shot 5 light bolts simultaneously at double damage and a second wand that always cast reinvigorating circle, so I had an infinite use healing item and an amazing weapon.
I put electric torch on my light wand, thinking "I'll use this to light things up if needed"
Little did I know that the game will sometimes apparently give you an instakill electric shock if you touch something metallic with it. I dropped from full health to 0 instantly, with electricity as my cause of death.
Noita