There are really two primary strategies for this:
1.) Take out his armour with the bombs, and then while it's down, hit him with enough damage to kill him. This may take more than one cycle, and his regeneration is not a lot, but he also does a fair bit of damage. Degeneration combined with damage works better on him than damage alone, so consider bring a Mesmer, Necromancer, and/or Ranger who can give him conditions that will reduce his health. If you're doing hero/hench, this is really your only option.
2.) If you have more than one human player, you can have someone (preferably someone with high HP) running bombs to him the whole time to keep his armour consistently low, rather than having everyone stop and do damage. The downside is you lose one damage dealer, but the plus side is that you can get a continuous rhythm going without stops and starts, which may be more efficient.
So you could play around with both of those ideas. The second worked much better for me, but it depends on your team and your style of play.
Another suggestion I would make is to bring something to reduce your DP if you find yourself dying a lot. If you have 60 DP, Cyndr's damage combined with the fire in the area will kill most characters off too fast for them to do any significant damage in a cycle, making the task borderline impossible. If you don't have DP, it's a
lot easier to withstand the attacks and get the job done-- so get rid of your DP if you can.
It's a bit tough, so keep trying and don't give up.
