That step took me maybe 15 minutes to complete. Pick middle tree Sunbreaker. The charged melee on that subclass is a mini throwing hammer. In any lost sector it will one hit kill any red bar adds. You throw it, get a kill and pick up the hammer so you have an indefinite charged melee. Again, literally should only take you 15 minutes to knock that one out by farming an add heavy location such as a planetary lost sector. To be clear the requirement is asking you to get charged melee kills, so regular non-charged melee kills do not count for progress. That is why picking middle tree Sunbreaker works great as you can use it over and over as long as you pick up the hammer after throwing it to have a constant charged melee.