Here is my 2 cents to elaborate on a quicker and more efficient way to boost this achievement. As CaptHowdy mentions, boost this in a RANKED Supremacy match and don't worry about the control points. A group of 4 boosters is the ideal size because it will be the quickest. Have all 4 boosters create a mech that has a repair backpack, 2 burst machine guns, and the all weakest body parts. DO NOT have weapon skills attributed like Syphon Shot or Corrosion. Leave these empty.
Each team will boost 20 kills then rotate to the next player. Team 1 will consist of Player A and Player B. Team 2 will consist of Player C and Player D. All boosters are to meet at a predetermined location. Player A will get the first 20 kills. Player B will follow then C and finally D. Player A shoots Player C until Player C's life is below 25%. Player D will then heal Player C (by pressing the B buttton). Player A will then shoot Player D until Player D's life is below 25% and Player C will heal him.
When you heal someone that has 25% or less health, you will receive 100XP for Savior, which is healing an ally with low health, and you will also receive 50XP for healing an ally at all. Once Player C and Player D have both been healed, Player A kills both Player C and D. This cycle then gets repeated until Player A gets 20 kills.
20 kills in a row is a Legendary Streak, which is worth 500XP. During this streak, you will get a streak at 3 kills (50XP), 10 kills (100XP), and 15 kills (250XP). Then the process is repeated with the other 3 boosters. You can boost for as long as you like, just keep the increments to 20 kills.
Supremacy matches have 15 minute time limits. As long as the score is even at the end of that timeframe, overtime will occur. Overtime is 3 minutes in length and will repeat indefinately, as long as the score remains tied.