...an angry titan?

Zappiel said:

Ha, indeed! An angry Titan that goes for the psyker that annoyed it first.

If you are in a highly urban setting you could collapse a building on it. Won't take it out, but it might reduce it's mobility to zero, making it a sitting duck. Or if there are underground structures, do the same.

When my Kill team dropped the Cerberus Valificant they blew out the void shields by redirecting a chaos artillery battery onto the titan. The nearby battery was actually their intended target, and it turned out that the forces of Chaos had been making precision artillery strikes by slaving all the guns to a single rogue psyker suspended in an amniotic tanks and attached to servitors, gun crews, auger arrays, and firing mechanisms by a forest of psyactive cables. That all proved to be a big mistake when the Librarian successfully used Compel to force the psyker to shoot the titan with the battery of big guns. Two salvos did the shields in, despite plenty of missing. Of course, the Titan then vaporized most of the battery with the massive blast radius of its Plasma Blastgun, but since that was actually an objective for the killteam they could hardly complain.

the classical way to fight a titan is close and personal with melta bombs, or Land Raiders or thunderhawks/stormraven.