Okay, so let's talk a bit about the Achilles.
I think the Achilles is a balanced ship in Starsector, it only became broken in the tournament.
Some ships have great synergy and become stronger in number, it's even more apparent with missile boats. By destroying ships, you remove some of the firepower of the fleet, the purpose of the missile boat is to do this at a safe distance. Its biggest weakness? It can't do that fast enough if it's outnumbered, which will eventually happen in the campaign as you can't equal the size of some of the biggest AI fleets. But everything that can't outnumber your missiles boats is toasted.
I took advantage of that in the tournament by picking the Luddic Path with its -50% cost on low-tech ships. By doing that i ensured no one would ever be able to outnumber me, at least in the first round. The only thing that worried me were some weird fleets that could ignore the monstrous lrm spam. I thought about 4 possible weaknesses for my fleet.
1: Exigency and AA. The repulsors aren't that big deal because there's a cooldown between each use, the cooldown would be long enough for me to vaporize the target. Even the reflected Lrms would have been intercepted by my own Lrm curtain.
What really worried me was the ball-lightning PD that Exi use as it is incredibly efficient against swarm of stuffs, including missiles. I think DR said the ball lightning gun lacked the punch to destroy lrms instantly so i'm not sure but in theory Exi/AA should have been able to endure the storm long enough until my Achilles ran out of ammo and ends up defenseless. That would be a very gimmicky build though, very unlikely someone made that for the Tournament.
2: Phase ships. One of the big weakness of missile boats is the missile fuel, sometimes they chase around some frigate and ran out of juice and can't maneuver anymore and explode on contact with anything, including your own ships. That's one of the reason missile boats aren't that strong in campaign because they need the overly expensive ECCM package hull mode that ensure your missiles will hit something before they ran out of fuel and possibly destroy one of your ships, but even with ECCM it can happen.
That's why phase ships, frigates especially are terrifying to fight against when you use a lot of Lrms. A phase ships can't stay cloaked for a while, letting the Lrms buzz around until they run out of fuel. Imagine a bunch of Afflictors skulking around next to your missile boats, baiting the Lrms and waiting them to destroy their own fleet? it sounds gimmicky but it's the worst nightmare of a LRM based fleet.
3: Carriers. One of the other big weakness of the Achilles spam fleet is vision, they can't launch their Lrms if they can't see the enemy. A bunch of Condors hugging the edge of the map will never get mauled by missiles because they stay out of vision. Your fighters will still have to go past the thick LRM curtain but hey, that's what Talons are for right? This fleet isn't perfect for the tournament as the AI may or may not make stupid positioning decisions with its carriers, also my Achilles had pretty heavy flak weaponry just in case.
4: Phase and carriers. Blackrock's Serkets would have been atrocious to fight.
But yeah, even thought the Achilles fleet have weakness, it's still very, very strong against everything else. I think it's strength come more from the -50% cost from the Luddic Path than the Achilles itself. Hell, my cruisers were as cheap as destroyers! With the normal cost, i would have been able to field only 6 Achilles (maybe 5 if we count the crew and the Hound scout), much, much less threatening.
In any cases, i will change my fleet. But in the campaign? The Achilles is definitely fine.