Coursing Through You is an achievement that is completed by defeating Telos after having taken the full damage of the red, green, or black virus in the fifth phase. The fifth phase is unlocked when Telos is fought at 100% enrage or higher. It is recommended to fight Telos at 100% enrage to avoid taking higher damage than necessary.
Despite the achievement's wording, it is not unlocked by defeating Telos while the virus is active on the player.
The virus lasts for 30 seconds and deals significantly more damage than in previous phases. As with those viruses, tanking it completely also deals 10 times the bleed damage on the final hit, so players should activate Barricade or Reflect when the virus has 10 or less seconds remaining.