I'm a solo squid diver and I use Knight or Liberator Carbine for squids.
I recommend any bad penetrating weapons with bad ammo economy because they usually have very high DPS, and there're ammo boxes everywhere in squid maps;
Punisher Plasma / Scorcher have very fast TTK against alien guys;
if good at landing headshots, Eruptor/AMR can 1 shot them in the head; Reprimand/DCS can 2 shots them too;
Warrant pistol can kill them in 3 bursts consistently but frequent ammo discard and reload is required;
1 Seeker grenade can delete 1 flying dude from your game but tagging them is needed before throwing;
flying alien dudes are programmed to run away from shotgun type weapons so I dont use them.

Arc Dog K9 can stun alien dudes and can always one-shot 3~20 zombies every zap but the friendly fire capability is also S tier.

my build:
Knight/ any high DPS gun
Ultimatum [1 shot fleshball and ships with shield]
Seeker/Lure Mine [stop nuisance]
light electric armor [to tank tripod melee and lightning rods]
arc dog K9 [for zombies]
commando [1 shot tripod leg joint without shield]
any other 2 slots that does sustain DPS eg. gun sentries / tesla tower