For titans you want to stack heroes of the strong color against the titan. For e.g. against a red titan, you want to bring as many blue heroes as possible.
This means you’ll have at least 5 different titan teams, one for each color of titan. I keep all of my teams in a spreadsheet instead of buying team slots, although the latter is more convenient despite costing gems.
Best heroes are those that have high attack stats, or have some good secondary effect (either buff attack, drop titans defense, heal you, etc.)
Definitely max out Wu Kong. He should be in every titan team, except against the Alpha Gryphon. Grimm, Boldtusk, Kiril, and later Athena, Elena, Evelyn and Frida are all great against titans too.
Some further reading regarding titans: Titan Attack Fundamentals - (new player Titan guide)
My advice though, you want to max out some 4 stars first, before your 5 stars. They’re a lot faster to level and they don’t require super rare ascension items like the 5 stars do for their 4th ascension. In terms of stats, 4stars at 4/70 > 5 stars at 3/70.
Ideally you also should do a 3 star team before 4 stars… but it seems kinda pointless now for you since you’re so far ahead. It would only be for those events that allow 3 stars, so it’s your call if that’s important enough for you