It is possible to get multiple shards from the same boss by entering the boss room from Out Of Bounds after you have already beaten it. Reaching a total of 8 shards will remove the barrier, so it could be used to skip bosses if there is a fast method of doing this.

It impossible to pick up a 9th shard.

I was able to beat the game without killing the Aqueducts 2 boss, and it says 100% on the save file.

Very big find. There are only 4 bosses you could skip with this, as the main-stage bosses are all required to get to Gael's Castle. The time we could save in 100% for skipping each boss after collecting all the collectibles are: Underground Temple: after you collect the heart piece is about 1 minute 20 seconds. Dark Forest-2: after you collect the armor is about 1 minute 10 seconds. Aqueducts-2: after you collect the heart piece is about 25 seconds. Frozen Mines: after the final backpack upgrade is about 2 minutes.

The biggest time-save would be Frozen Mines, as it would skip the entire block puzzle before the boss. This means that in order for this trick to save any time, we would need a setup and a boss fight that in total takes less than 2 minutes to collect the extra shard on.

However, this is huge for any% as it means you only need to progress through the strength, underwater breathing, and dash upgrades. Demon Armor and Level 3 sword will be unlocked via debug room, while double jump and cape (though we'll unlock it in debug anyways) won't be needed anymore. Won't need to visit Aqueducts-2 or Frozen Mines at all in the run. Normal any% glitched time could be sub-30 minutes real-time with this.

"Aqueducts-2: after you collect the heart piece is about 25 seconds." You could also clip into the heart upgrade room then save and quit out, which would skip the puzzle.. but idk if it would save time

I also found a way into the aqueducts 2 fight, but havnt tested if the boss respawns. (I miss the clip in this vid, but this still shows where the boss room is)

Found Dark forest 2's boss room, i softlocked myself by jumping too far, but it did start the fight

Just got into DF1's boss fight. Got into it very quickly, so this may be the best boss to use From the top left fire walk with me statue, i downward zipped, and landed near the strength upgrade, then dash jumped right.

It is possible to zip directly into the boss room, but i cant trigger it. Maybe the camera being locked is preventing the boss from spawning

Found a faster method. Got into the boss room in under 30 seconds. Starting from secret entance.

Haha was just coming to post just that, though mine uses the shrine by the secret entrance.

So this is fast enough that it should get rid of the Underground Temple, Dark Forest-2, and Frozen Mine boss fights (and frozen mine puzzle) in 100%. The Aqueducts-2 fight is still faster to do, as long as you get the 1 cycle kill.

In Any% this is going to completely remove all sub-stages except for Dark Forest-2 for the underwater breathing upgrade, and Underground Temple for the dev room.

Edit: Alright so we both found the same path from the secret entrance- seems like there couldn't be any faster way.

Assuming every shard only takes 45-50 seconds, in 100% this will save: 20-25 seconds on Dark Forest-2 30-35 seconds on Underground Temple 70-75 seconds on Frozen Mine

Overall shaving about 2:00-2:15 off. Bit of a shame though, watching the same fight 4 times (3 of them being back-to-back) is less interesting than the previous route.

Any% on the other hand barely has anything to worry about other than the 4 main stages. You will play through the first few stages as normal, going to Dark Forest-1 to grab the underwater breathing upgrade before going back to the map and finishing the 4 main stages. Then you will hit the debug room in Underground Temple, go back to map, and farm the Dark Forest-1 fight 4 times in a row before heading to Gael's Castle.

The possible times for the glitched categories are starting to get very low.

Veteran any% would also save a lot of time with this trick.

Since the upgrades are in different places, the debug room is accessed a lot earlier than a normal run.

Glenfinnan > DF1 (spears) > Glenfinnan secret exit > Underground temple (debug room, elevator) > Dark Forest-1 boss fight x5 > Aqueducts-1 boss > Devil's Peak boss > Gael's Castle

Also, small optomisation: saving and quitting to world map after collecting a shard will stop the map location from changing. This would save around 3 seconds for every duplicated shard/boss

.. Collecting 8 shards before unlocking Gael's Castle will make the game impossible to complete. It was kinda obvious, but I tested it anyway.