You should move around when facing a bomb king and if he gets you low then try and get near your team, BUT don't stay too close since he does splash damage and it could hurt real bad.
- Try and get used to Bomb King's trajectory, both with and without Accelerant. This will make it much easier to land hits (Or headshots!) on him while he's midair.
- 2 direct hits from Chain Reaction will leave you with 130 health. As such, it is impossible to kill a good Bomb King if any allies are nearby. Isolate the fight if the Bomb King has Chain Reaction.
- Bomb King is not hard to hit. Any strafing side-to side will amount to nothing due to his general ... fatness? Width? Try and aim left and right, rather than up and down like most humanoids.