If the target player is no longer a legal target as the spell resolves but the creature is a legal target, the creature is dealt damage, you gain that much life, and you amass 4. No player moves cards from their library.

If the target creature is no longer a legal target as the spell resolves but the player is a legal target, no creature is dealt damage and you gain no life, but the target player still moves the top cards of their library and you still amass 4.

Abilities that trigger while Enter the God-Eternals is resolving or that trigger as the target creature dies are all put onto the stack after the spell has finished resolving and the target creature has (if lethally damaged) died.

If the target creature is dealt lethal damage by Enter the God-Eternals, it'll still be on the battlefield while you gain life, the target player moves cards, and you amass 4. Any abilities it has may affect these actions and may trigger on them.

If both targets are no longer legal targets as the spell tries to resolve, it doesn't resolve. You don't amass 4.

If the target creature has less than 4 toughness, Enter the God-Eternals still deals 4 damage to it and you'll gain 4 life.

You can't cast Enter the God-Eternals without targeting a creature and a player.

To amass Zombies N, if you don't control an Army creature, create a 0/0 black Zombie Army creature token. Then you choose an Army creature you control and put N +1/+1 counters on it. If that Army isn't already a Zombie, it becomes a Zombie in addition to its other types.

In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Zombies, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Zombie, it becomes a Zombie in addition to its other types.

Some cards refer to the "amassed Army." That means the Army creature you chose to receive counters, even if no counters were placed on it for some reason.

Amass abilities are now written as "amass [subtype] N." Previous cards with amass have received errata to say "amass Zombies N."

If you don't control an Army, the Zombie Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.

Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. If the Army creature you chose isn't already an Orc, it becomes an Orc in addition to its other types. By combining cards with amass Orcs and amass Zombies, you can end up with an Orc Zombie Army.