Valen has to do with red titans. Valen is blue, so against red titans he will always be part of your team until you get better blue def debuffers (Grimm or Isarnia for instance). Even when Tiburtus is above 3/60.
For the other four colours, a strong Tiburtus is the better choice than Valen as long as you don’t have a red defence debuffer (Gormek, Wilbur,…) to use against green titans, a green one (Costumed Brienne, Buddy,…) against blue titans, or a yellow one (Costumed Vivica or that Wonderland rabbit) for purple titans.
In future you will, of course, get more and more defence lowerers and heroes with great effects useful for titan fights. So, your titan teams woll constantly improve and evolve.