Video is part of the 3rd mirror level and the boss fight as that's the hardest part of the mirror mode in my opinion.
General tips for mirror mode are just to replay the levels in normal mode at least a few times each as every time you die in mirror mode you have to restart from the beginning. All mirror mode does in flip the direction you're moving. All the puzzles/platforming are exactly the same. All the achievements for beating the bosses without dying have to be done in normal mode too so you might as well get those all done before attempting mirror mode.
3rd Boss fight starts about 2:15 in the video if that's all you're looking for. Same tips apply that poolsharker and memories mentioned for beating it without dying. The patterns are random but the phases play out the same way.
First phase is exactly the same as normal mode, just aim at whichever triangle is facing inward.
Second phase is the hardest in my opinion as your first shot will now spin counter-clockwise 90 degrees as opposed to clockwise in the normal mode. The 2nd shot still spins 180 degrees so you still just need to aim opposite your target same as before. But re-training your brain to aim properly in mirror mode can take some practice, especially as you can't pause the game to think about your next shot. Just try to think of it as a clock and always aim in front of the target on said clock. If the target is at 12 o'clock (up) then you want to aim at 3 (right). If it's at 3 (right) then you want to aim at 6 (down), etc.
Third phase is much easier using the pattern MemoriesOfFinal mentioned in the other solution and it still works exactly the same way in mirror mode. Start the phase by hitting the top square with the 180 degree spin shot. Then from that point on the pattern is:
At (point your arrow where the killer arrow was before it moves)
Away (opposite of where the arrow was before it moves)
Just knowing the pattern for the third phase is probably more useful than the video itself as you'll have to memorize the third level in order to attempt the boss fight enough times to beat it. Probably took me about 2 hours total just working on the 3rd mirror level after having done everything else already. Maybe 6 total for everything.