|95% of Spell damage resistance from spells + Unliving|
See the Comparison of Level 6 Creatures.
Tactics and info
They have very low defensive stats for a level 6 unit, the lowest health value if we exclude the Enchanter and also the lowest defense.
The Diamond Golems also have the lowest attack and damage values of a level 6 unit.
Efficiency and cost
Because they can be recruited pretty easily after taking a Golem Factory, and also due to their very low cost, they are units worth having in early game. However, later on they serve only as cannon fodder.
There aren't any necessary tactics for them. Use them as you see fit. Keep in mind that they take only 5% damage from spells (95% Magic Damage reduction).