First you go here
Then wall jump up then you should be here
Then walk across the log staying to the left to avoid invisible walls
and you should be here
Keep going and when u get here you have to jump as far as you can. Try not to get stuck in between spikes(although you can still make it if you do)
Then you should be here after making the jump. once u are here just jump down and there you go.
Now you can jump right through the wall and grab flag.
I hope this method hasn't been posted . i found it out after i tried the old way and i was unable to do it so i found this way. I know this is old news getting back there just sharing my way and im pretty sure it hasnt been posted but if it has please tell me.