So I’ve noticed that people with a bit lower team score seem to deal more damage to titans on average. I wonder if they’re using some better items or if it’s cause my characters are just no good for that purpose?
They are:
Kelile
Li Xiu
Melendor
Kiril
Balthazar
There’s a massive variety of factors that go into titan fights. Team setup, survivability, defense reduction, items, smart gem use, etc. I’ll tell you one thing that has absolutely nothing to do with your effectiveness in regards to a titan, though; Team power.
This should not be your focus. Your strongest heroes may in fact not be your best possible option. Now, I have no idea what options you have, if any, in the flexibility of your lineup, but I will tell you some basic things that helped me earlier on.
Minor mana potions. Small ones. The ones you probably think are useless (as I did) They’re incredibly cheap to farm and create, and you can bring 10 of them into battle.
Arrows/axes. Pretty similar to mana potions. The longer all your heroes survive, the better off you’ll be, obviously.
As far as team goes, multi-hitters are at a severe disadvantage in titan fights for the simple reason that they’re designed to specifically multi-hit. Meaning less damage on their specials and rarely a useful ability attached to them, as it was intended to be spread out over numerous opponents. There are exceptions to this. Grimm/Athena/Gormek/Tiburtus are incredibly helpful if you have them. Defense reduction on titans is a game changer, at least for that aspect of the game. Also hel for mana shutdown, but I would assume she would be in your lineup if you had her, so I won’t go into that.
Wu kong. If you have him, or can get him, I would strongly recommend it as he will exponentially increase your damage. Moreso on pure gem use than specials.
I’m also a fan of offensive use riposte vs titans, especially when there’s limited options and said titan is much stronger than you can handle. They all have a multi hit special, so when that hits three of your riposted heroes for 6-800 each, it’s about 2500 potential damage for free. Which is far stronger than just about any special there is. This kind of thing is where the mana potions come in handy. Spam them to full for your riposte hero just before the titan fills its mana and watch the fireworks.
But anyway, I didn’t intend to write a novel. I’m sure I’m missing a bunch, and others will have useful suggestions. I’ll leave it to them.
Novel is great 
I’ll keep those notes and hints things in mind.
I have 1 other hero only, and he seems much worse there - Skittleskull. I don’t even have a 4* purple, not to mention Hel 
But if I’ll find someone with the riposte, even a 3*, I’ll make sure to keep them around.
Your team could be better. 2 healers is a bit too much on titans, unless you struggle to stay alive. But survivability can be easily helped with items like axes or arrows if you can do axes. And banners.
Like TB said, hit all heroes are bad on titans, as there is only one target and they are slow. So Li Xiu isn’t a good choice. Ideally you want one hero to boost your attack, preferably Boldtusk but Kiril is ok too. One to lower titan defense, Like Athena, gormek, grimm, tibertus or any lower * I don’t remember which ones. Then rest fast single target heroes.
Kelile and Balttazar are ok, but I would look at replacing Li and Melendor for titans.