1) Engage him and DPS until 10 seconds before shadowbolt.
2) Entire raid runs out, leave only paladins there with MT.
3) Paladins bubble (Divine shield) themselves and top up MT through the shadowbolt volleys.
4) Raid runs back and finishes the fight
5) If more time is needed, 10s before 2nd shadowbolt volley... All healers stay in and help heal through the shadowbolt while 1 shaman/priest runs away with the raid and chain heals / COH them.
This makes him killable even if you have only like 15-20 players in your raid.