If the Imperium knows the location of the Titan then so does the Mechanicum. Make sure to include Skitarii, Protectors, Praetorian Guards. Also supporting the fleet should include an Ark Mechanica. Instead of Imperial Guard use the Tech-Guard (same thing really just the exclusive guardians of Forge Worlds) they have better equipment and bionics. For fluff purposes include Space Marine Chapters with strong ties to the Mechanicum like the Iron Hands and Salamanders. Of course you could make up your own Chapter(s) to fill in what you need.
In some cases it is best to have such massive combats to be setting elements with smaller elements the PCs can handle. Like have them pilot a boarding torpedo across the void to sabotage one of the corner stones of the Imperial Fleet with the larger battle happening around them. Unless of course they want to be in command of a ship(s) then Rogue Trader would be needed for the void warfare.
The fun ground combat could have them supported by Baneblades, Stormblades and other Titan killing weapons. Again such weapon systems should be set pieces only. But over a fully functional Warlord? Your looking at something the Mechanicum would start a Crusade over. Give it something important like STC pattern never before seen, regenerating armor, anything from the Golden or Dark Ages of Technology. But sometimes characters can be eclipsed by the sheer extent of everything that is happening around them.