Leviathans are badly designed
1 they require AT lvl 5 weapons to deal with which are impratical against all the other enemy types.
2 while other superheavies like tanks and impalers require reinforcements to be called or are waiting at outposts or just become a single unit patrol, leviathans just keep spawning randomly troughout the mission without taking place of patrols and no matter how much you avoid alerting the watchers, leviathans will always be there to annoy and snipe you.
3 you may say that the new changes makes them EASIER to avoid, but, objectives that require to stand still have not gotten easier, with AT emplacement i can shoot them but they can shoot me too, everytime i place it down it gets destroyed as soon as i start shooting, and when it doenst get destroyed after killing two leviathans a third one appears while the strategem is on cooldown and the one i used does not have ammo.
4 some say strafing runs and gatling barrages can kill them, but its very dificult to align the shots and its no a guarenteed kill because of the leviathans movement a the strategem just missing, and to kill with these strategems they have to be very close which means getting pushed around by those annoying shots.
tldr; they are annoying, tanky, require too much effort for little impact and shouldnt spawn the way they do.
with that, i have some suggestions on making them less unfun to deal with;
1 lower armor pentration, AP 4 instead of AP 5 would make HMG, AMR and laser cannon worth bringing without feeling like its not enough firepower, this makes them less awkward to deal with since heavy weapons can be used against harvesters and overseers.
2 limited spawning, if leviathans were a secondary objective like factory striders convoys or protecting objectives instead of a mission modifier, it would make them feel less like a bullshit thing that appeared out of nowhere and more like something to be careful about and be planned, killing them is rewarding because the place is now less protected but AT weapons are unpratical so are only used for them.
3 killing one puts them on cooldown, just like my AT emplacement is on cooldown the leviathans spawning also is on cooldown, rewards the player for killing them but still makes it impratical to bring AT wepons.
Ive been playing this game since the TCS missions and play on d10, i think either one of these changes would make leviathans less of an annoyance and more of a threat to be dealt with.
I know the developers wanted to make it something to be avoided, but this is not how to do it, for example resident evil 3 where nemesis can be avoided to not die and waste ammo, or be fought to obtain a big reward, with leviathans, either kill it and waste all your ammo or dont and die trying to do the objective, this is why they are badly designed, always all over the map no matter what the player does, and killing one is worthless because another one will appear right after.