Be warned: this makes adventure mode ridiculously easy unless something stupid happens to you like you're knocked into an inescapable pit in the middle of battle, or you're caught in freezing water, etc.
In Fort mode, the most expedient danger room design is an area of upright wooden training spears linked to a lever, which is manually operated by someone other than your soldiers. Since there's no way in Adventure mode to tell your companions to pull a lever, this is impossible to use as an adventurer.
At first I thought that if I simply built a repeater using floodgates and pressure plates, I could have automatic spikes that would function in aventure mode. This is not necessarily the case:
If you go into the init files you can toggle whether or not traps work in adventure mode. This apparently includes pressures plates, necessary for the spears to operate. With adventure mode traps turned off the spears just sit there in an upright position, and with it turned on something interesting happened. The spikes triggered once my adventurer set foot in the room, causing him to block/dodge and gain skills, but the spikes never went back down again. There may be some delay issues with flowing water or just something I didn't expect, but I dunno.
So instead, I built a second disposable fort and built this:
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The top and bottom are walls, and the rest represent upright spears and pressure plates. The plates are set to trigger when a dwarf or larger walks over them and citizens can trigger these. I'm not sure if that last thing makes any kind of difference in adventure mode but whatever. The corridor is a bit too long, as the spikes trigger once you walk 2-3 spaces away from the plates but this isn't a bad thing, because this also trains any companions you may have with you.
(EDIT: the upright spears I used had only 3-6 spears per tile, and this should go faster if you build more.)
It took me about 15 minutes walking back and forth over this thing to train up to legendary, and maybe another 15 minutes to see how high I could go up in some skills. I got Fighter up to like xxx/4100 or xxx/4300, or something. And then I became Killy McGee until I fell into an evaporated pond in a village and didn't want to go through the process of luring every villager to the edge of the pond, slaughtering them, and then traveling out of there.
What this does:
1. Projectiles are no longer a serious threat. I was able to block or dodge anything fired at me, and I was able to do things like not worry about 5-6 archers firing at me while I did battle with the other guys, and storm castles unscathed where normally I'd get shot in the leg or hand despite armor, leading to fun.
2. Your companions are much more effective but still sorta stupid. Eventually doing things like walking up to a group of goblins and fighting all of them while mostly unarmored and having the parts that are armored covered in copper will catch up with them, leading to serious injury and death. But they last a lot longer and are amazingly effective.
3. If you fight intelligently you're almost immortal. If you only take on 1-2 monsters at a time and have decent equipment nothing can touch you. I did get hit occasionally, with hammers and maces and they would leave bruises. But for the most part you're too busy killing the things before they can kill you.
4. Combat is mostly unchanged. If you aim your attacks the difficulty is still there, but it's somewhat easier to target individual body parts. You can still miss enemies, and miss enemies with aimed attacks. It's significantly easier to damage the heart and lungs with aimed attacks, limb removal is somewhat easier (needs testing with non-steel weapon) and breaking bones with the pommel of your sword is trivial. Enemies still hit you, eventually, especially if you do things like lie down in a room full of knife-wielding angry villagers. This could present a problem, but for a couple rounds of combat you're fine.
5. Your stats (strength, agility, musicality) don't increase alarmingly. I did notice small stat gains, but this was equivalent to chopping up like 30 enemies in a fair fight. And by this time in adventure mode somebody with a spear gets lucky and severs your spine.
6. Keep in mind enabling adventure mode traps applies to all traps, not just this thing, so be advised that you could still have your face crushed by spiked bronze balls or something, but this, again, needs testing, and may be something your uberadventurer just dodges.
This needs more testing and stuff, but enjoy