Im assuming its a 24 word seed so should only need 18 words if the first 6 are in order. So you would need test 20!/2! combinations. Which is around 2^60 and should be possible since the bitcoin puzzles have been solved up to 2^65. Also the seeds have checksum bits which could reduce the total combinations to test down to 2^52. 24 words out of a list of 2^11 words is in total 2^264 of entropy and 8 bits of that is a checksum. and If you have the public key you might be able to use pollard's kangaroo algorithm to solve it in 2^30 time but I don't think that algorithm would be possible in this instance.