The Guardian is a master of defensive combat. Their damage output is lower than the other warrior classes and they themselves will get completely shot to bits before they can do anything, but the Guardian class provides defensive perks to the units assigned to the Titan, typically focused on troops of the same race.
When using one either on attack or defense, just don't bother putting on any Relics that boost just the Titan; focus entirely on relics that boost the relevant Troop. This can allow for a very, very powerful army. One particularly powerful combo is a high-grade Ragnar Guardian with good buff types (i.e. not Resist Lightning ones) supporting an army of pure Goliaths with every Relic also boosting the Goliaths, allowing for a massive wall of club-wielding health to slowly but surely stomp over enemy armies.