Regarding the First level with the Mirror
If your playing in Mirror Mode (Need to get to level 97.1 to Unblock the Mirror Mode Option) There will spawn a Key there, This key goes to a door on level 62.
Regarding 23.3
Try kill the Banana, Then the Green Blob that Comes, Throw a Normal bomb down to the Bombinos so at least one will die instantly. Then jump over them (if you can) And get the Blue Bombs OR use the Black Bombs (Tricky and you need to Time it good to kill them) After that just use the blue bombs to knock out the Cherry and the Last Green Blob.
23.4
Is a bit Trickier, You need to throw bombs to knock out the "What are they called?" (Red "Furry" Balls) While paying attention to the Saws. Then you need to get the Lemon out, That is just trial and error. Its very frustrating sometimes to knock him outside his Little Box.
23.5
Enter from the Left side, Pay attention to the Saws and time your jumps good. Get the

and leave with going to the next level on either right or Left side (Middle is dangerous).
23.6
Step on the Red switch and make your way to the Portal that appears. If your able to Kick Bombs upwards this level is easier, Freeze the blades and time your first jump over to the middle platform and then quickly jump to the lowest platform to the right if your able to upkick bombs, Else just jump quick and exit.
Hope this help,

Elvarion