Caffeine OD on painkillers will just not work based on the number of pills you'll have to take for the caffeine level in your bloodstream to be lethal. Remember that your body is built to prevent dangers as much as it could, and there's this glob in your stomach that will make it so much harder to absorb the pills before you throw them all up.
Also, energy drinks won't work. You'll have to drink A LOT of them, and there's a high chance you'll vomit before getting anywhere near the lethal dose. There were cases of people that accidentally died, but they are extremely rare.
I made a thread about my experience on caffeine OD. If you pain tolerance is high, then it's not impossible to ctb. Otherwise, I won't really recommend it. My pain tolerance is really high, and I still suffered from a great deal of discomforts. The most reliable way to OD on caffeine would probably through powder I'd say.