At 10% health, the following things happen:
Archimonde gets attacked by Wisps, which kill him automatically after 60 seconds
Tyrande gives the raid a 60 second invulnerability (de-)buff (Protection of Elune)
Archimonde channels Nordrassil, the World Tree (just an animation, no other effect). He also spams Finger of Death to no effect
Since reaching Phase 2 is an automatic victory, the only thing the raid has to accomplish is to bring Archimonde down to 10% within 9 minutes. This means the raid has to deal 10% damage per minute, which is 500,000 damage in 60 seconds, or less than 10,000 DPS. Given 15 damage dealers, this means an average of 670 DPS per damage dealer, which is less than half of what players at this gear level should be able to sustain.
Therefore, nobody should take any risk. Running away during the whole fight is perfectly ok (because it means that the people on the other side do their job unimpaired) - just don't die.