My oponent has a crab and a Ferrous Forerunner. If I attack with 6 might, I can kill both, or I need 7 might to assign 1 in crab?

To address your question about combat damage assignment, we must first look at the stats of your opponent's units:

• Ferrous Forerunner: 6 Might
• Scuttle Crab: 0 Might

1. The Minimum Lethal Requirement

As established in FAQ #10658, 0-Might units do not die just by existing. They require 1 point of damage to be marked on them to be considered lethally damaged and killed.

2. Can you kill both with 6 Might?

No, you cannot kill both with 6 Might.

According to Rule 465.2.c.3 and FAQ #1675, you must assign lethal damage to a unit before moving to the next.

• Targeting the Forerunner: You assign 6 damage to the Ferrous Forerunner. This is its exact Might, which constitutes lethal damage.
• Targeting the Crab: Since you have already assigned lethal damage to the Ferrous Forerunner, you have fulfilled the requirement to move on to the next unit, but you have 0 Might remaining to assign to the Scuttle Crab.

Crucially: You are not required to kill every unit on the battlefield to win combat; however, you only conquer the battlefield if the enemy has no units remaining after the Combat Damage step.

• If you assign all 6 damage to the Ferrous Forerunner, it will be killed.
• The Scuttle Crab (with 0 damage marked) will remain on the battlefield.
• Result: You have not cleared the battlefield; therefore, your units will be recalled to your base during the Combat Cleanup.

Summary

• To kill the Ferrous Forerunner: You must assign at least 6 damage.
• To kill the Scuttle Crab: You must assign at least 1 damage.
• Total needed to kill both: You need 7 Might (6 for the Forerunner + 1 for the Crab).

With 6 Might, you can successfully kill the Ferrous Forerunner, but you will not have the remaining damage pool necessary to kill the Scuttle Crab, meaning the Scuttle Crab will survive and you will not win the battlefield.