When you fight Titans, it will show weak points(for extra damage). If you can hit it with a combination of 3, no matter the color(even if you have the hero or not), you will stun.
As a person with a team of 3.5K after 2months - and also as person who learned by doing very expensive mistakes:
This will give you the best headstart. After 3-4 months, you should be fully upgraded. And ready to spend some gems on Atlantis.
Now that I mentioned Atlantis - it's a really important, where you can get the best heroes. But more importantly, the loot you get in Season 2 is increased as well during the event. So, if you can farm any stages in S2 - I will strongly advice you.
As for raiding - you can't put heroes in random order. Think like the attacker. If the tank is a red hero, flanked by 2 other red heroes, the attacker would just stack on blue. So, you could have a red tank, and put other colours as flanks. Or, red tank and green flanks(as if the tank dies because they stacked blue, they would be weak against the green flanks). Always consider these things, at you will understand the concept in no time.
Check these links out:
https://docs.google.com/spreadsheets/d/1ADv2D76uQuvrk4pVW6CD7mI88GALMMHGUNfjoRBHO9c/edit#gid=0