Right, so my team is looking better than it was compared to Horus + multicolored slimes now. I just wanna know if it's viable for the harder/Descended dungeons if I evolve and level up enough?
Leader:
Horus
Subs:
Amaterasu
Parvati
Siren
Dub Mythlit (gonna replace with Rainbow Keeper when he decides to drop)
Going non-IAP all the way so it's highly unlikely I'll be able to get gods like Susano/Orochi/Hades for my Horus team, so advice will be appreciated.
I think your team is ok for now. Siren and Parvati are good to have, but both take away one color which makes it harder to activate Horus's skill. Amaterasu would seem like a useful skill on this team, but it takes a long time to charge. Her stats are very good though and she's a relatively useful monster to level up anyway.
Some good farmable monsters you'll want to look into for a Horus team:
Green/Blue Mech warriors: these act as a poor mans Susano with a skill-up-able half damage skill and give you a ton of HP too. Rainbow keeper can handle this in the mean time.
Heartbreakers: Vampire is probably your best bet until you can take on Two Heroes or Valkyrie. Gigas would be fine but RCV is very helpful on this team. Max skilled is best.
Hera/Zeus: A little bit of a long term goal, but gravity is alway nice if you can stall for long enough to charge it.
Echidna: This can complicate things a bit since she is also fire, so you'll have to make up for it with an ultimate evo somewhere or maybe an Isis friend instead of Horus. Again, max skill is best.
Dark/Light Golems: Eventually you won't need this to get past some high defense enemies, but until then, these monsters have great HP and decent attack and can cut down the defense. A Lilith would be ok for enemies with low health, but she won't really help against Neptune since this team is not built for stalling.
I think that's the basic building blocks for a free Horus team. But your team should be good until you can catch a few of these.