One of the most trying quests in Anthem's campaign is the Legionnaires' Tombs quest that requires 3 multi-kills.
For some people, it's done very easily if they play Colosse for example. For others, this can be more difficult, especially for Interceptors. It appears that the key to validating is to kill 8 enemies within 10 seconds of each other, rather than killing opponents at once to validate it.
This means that by quickly killing weak enemies, you can get it. The best is to make an event that brings up many enemies in free and easy exploration mode and use zone techniques like your Ultimate or with a powerful weapon, for example a sniper rifle.
It is important that you complete the enemies, the participation does not count. It also seems that this action must be done in Free Play mode for this to count.
Using the Ultimate Colossus or a Pomegranate on groups of small enemies is a solution to consider if you really cannot do otherwise.