Does one of the heroes have a lot of crystals and gems seemly implanted in their bodies? Is this hero also a know it all who seems to be never wrong? Well sounds like you have an archivist problem, and as long as you have an archivist problem you have a heroes-not-being-dead problem.
So, what is an archivist? Basely someone realize that magical crystals and gems are a great with to store information, and memories, so they took a nerd with minor psychic powers and then jam as many shiny rocks into their brain as they could! And surprisingly instead of killing that nerd they turn them into the first archivist, of course this did take a few tries and a lot dead psychic nerds to get right but hey that progress for you. So, what does an archivist do?
They store information, all kinds of information, and then lets others have it. Not only is everything they have ever seen, heard, and read is store inside the many crystals inside their bodies but they can scan and store memories of other willing people as well. They can than share this information by not just telling people about but can show them it to in the form of minor illusions, or by using magical and tech devices design to receive data from them. So, they basely know everything and can anything? Not at all, first they do have limit to how much data, second just because they the data doesn’t mean they know what it means, or how to use it, they may not even know all the data they have as they store so much, they have to use a person search engine in their own heads to find out what they do know. They are little more than living computers in that way and often have the personality to match.
So, how do I kill them? Killing them is easy, they don’t really have any training or skills besides being a living database, the thing is you may not what to kill them. Thanks to their abilities archivist often have enough data to make them more valuable a live than dead. True, you can extract that data after they are dead, but you may damage some of the crystals and loss that data instead. Your best bet is to catch them alive if possible, heck into their mind, drain as much data as you can than kill them.