The Titan holds aggro; the Artillerist damages all; the Striker finishes elites before they overwhelm the tank.
Only place the Guardian directly on your vulnerable units (artillery, summoners, healers) if you are being overrun. This is a last-ditch shield wall. If you rely on this position, you have already lost the initiative. Giant Guardians Guide
In high-level play, keep one Guardian behind your front line for the first 30 seconds. When the enemy commits their assassins to kill your healer, then deploy the Guardian. This counter-engage flips the script instantly. The Titan holds aggro; the Artillerist damages all;