Legio Crucius here with 1 and soon to be 2 Reavers and a knight house of 4 indentured Gallants and 3 Crusaders; main opponent legion is Mortis with a Reaver & Warlord with one of all the forgeworld knights except the heavy shooting one.
The volcano cannon is the beginning and end of discussion; everything is about setting up that shot. As an example my manticore batteries and allied artillery blew their entire load just to knock 10
HP off the warlord to break its voidshields down one tier, and one good volley from just the reaver's volcano cannon then burned off 48 hull points, and would have killed it but for a command reroll. After that it's a big scary sure, but it can be largely ignored while sweeping up the thing's attendants.
As part of a larger game titans are set pieces and sweepers who rarely last 3 turns. The game really comes alive when they're gone on both sides and you have two shredded exhausted armies to smash against to the end. It really is down to you as a player to shepherd limited resources towards victory.
Just like a real war I guess
As for surviving a titan, bring your own terrain e.g. bastions and hide artillery behind it. Barring another titan, if a titan wants a unit dead, that unit is dead unless it is hitting on 5+. The only safeguard is swamping it with high value targets.