Allright: main problems in foundry is the great dmg that single groups do. If u run with SY, u won't have much problems.
Another counter are conditions/hexes...too many times groups fail for not having enough hex/condition removals. Bring a lot of them!
damage....Since Titans have great armor against fire and actually fire got the most AoE spells, u can either bring winter and an AoE fire spells elementalist, or choose something else like:
Mesmer: skills like Chaos Storm, Wastrel's Worry and many other Mesmer domination skills are useful....Energy Surge, Panic, Unnatural signet, Visions of Regret and many others spells are really good for they do armor ignoring dmg..
A necromancer is a good choice also...If u manage to ball mobs and spamm an SS/MoP u should be very quick.
Ritualist: why do everyone chooses SoS?? Plus on an hero that cannot even move them.....there are many other good skills in Channeling Magic, Clamor of the Souls for example is a great skill...Spirit Rift, Ancestor Rage, Splinter weapon, Cruel Was Daoshen, Gaze of Fury, Destruction....
Monks with Ray of Judgement and others AoE smiting skills, but this is slower imo...
Another problem is titans spawn after u have killed them...These can be hard to tank with heroes, since u should be able to ball, spike and kill, move heroes back and reball again....
Another little advise: know ur enemy....i mean look at those who are more dangerous for ur team-build..kill them fastly and prevent their actions. If u are an imbagon those pesky Margonite necromancer with Vocal Minority are ur enemies

Same for Rage Titans with Eruption which makes u blind.
If u are caster, those Anguish Titans are ur enemies: interruptions, spells stealing...