If I have an Elder Dragon and attack an opponent on a battlefield which has 5 4-might units, can I assign 2 combat damage each to kill them all since Elder Dragon has the lethal ability?

To answer your question: Yes, you can.

Because of Elder Dragon's ability, any amount of damage you deal to an enemy unit is considered lethal. Therefore, you only need to assign 1 damage to each of the five 4-Might units to kill them all.

The Ruling

According to Rule 460.2.c.2, "Lethal Damage is non-zero damage equaling or exceeding the Might of a Unit." However, Elder Dragon's ability ("Any amount of your damage is enough to kill enemy units") modifies this condition for your damage.

When you are the attacker assigning combat damage:

1. Rule 460.2.c.3 states: "Units must have lethal damage assigned to them in full before damage is assigned to a different Unit."
2. Because Elder Dragon makes 1 damage "lethal" for your damage, you satisfy the requirement of assigning "lethal damage" to a unit by assigning just 1 damage.
3. Once that 1 damage is assigned, the unit is considered to have received lethal damage, and you may move on to the next unit.

Sequence of Assignment

If you have a total of at least 5 Might (the sum of your attacking units' Might):

• Unit 1: Assign 1 damage (Lethal due to Elder Dragon).
• Unit 2: Assign 1 damage (Lethal due to Elder Dragon).
• Unit 3: Assign 1 damage (Lethal due to Elder Dragon).
• Unit 4: Assign 1 damage (Lethal due to Elder Dragon).
• Unit 5: Assign 1 damage (Lethal due to Elder Dragon).

As long as your total combat Might is at least 5, you can distribute 1 damage to each of the five units to kill them all. You do not need to assign 4 damage to each unit.