* Almost* all demon lord teams rely on using buffs so that he can't kill them.
Some comps use Wixwell to use shields; He's unique in that he can keep growing shields faster than demon lord can damage them. It's probably the easiest team to build, except that it depends 100% on pulling Wixwell.
An "unkillable" team uses either the block damage buff and/or the unkillable buff to keep everyone alive; precisely speed-tuned so that the buff is always in place when demon lord takes his turn.
Both types are commonly refered to as "unkillable" but it's an important distinction with Geo:
On a Block Damage team, he's a strong dps, most of his damage comes from his passive having a 30% chance each time hes hit to do 3% max hp damage
On an Unkillable team he's even better because he also reflects 30% of the damage DL hits him for.
The differnce being that block damage reduces the damage champs take to 0, whereas with unkillable they take the full amount of damage, but cannot drop below 1 hp.