I haven't been able to find a code for reducing your level to 1 for a new game +, and people were asking about it a while ago so I figured I would post what I made.
Shulk lv down
00CE43CF 00000001
04CE437C 00000000
04CE4380 00000000
02CE43E8 00000012
02CE43EC 0000001A
02CE43EA 0000000F
04CE43DB 00436600
Reyn lv down
00CE81A3 00000001
04CE8150 00000000
04CE8154 00000000
02CE81BC 0000002A
02CE81C0 0000000A
02CE81BE 0000000C
04CE81AF 0043BE00
Fiora lv down
00CEBF77 00000001
04CEBF24 00000000
04CEBF28 00000000
02CEBF90 00000011
02CEBF94 0000001B
02CEBF92 00000012
04CEBF83 00435E00
Dunban lv down
00CEFD4B 0000001B
04CEFCF8 00004ECB
04CEFCFC 0000000A
02CEFD64 0000010B
02CEFD68 000000A9
02CEFD66 0000004D
04CEFD57 004527C0
Sharla lv down
00CF3B1F 0000000A
04CF3ACC 00000000
04CF3AD0 00001200
02CF3B38 0000004E
02CF3B3C 00000057
02CF3B3A 0000001C
04CF3B2B 00446DC0
Riki lv down
00CF78F3 00000016
04CF78A0 00000000
04CF78A4 0000C000
02CF790C 000000AD
02CF7910 000000A6
02CF790E 00000038
04CF78FF 00456FA0
Melia lv down
00CFB6C7 00000017
04CFB674 00000000
04CFB678 0000A000
02CFB6E0 00000080
02CFB6E4 000000F8
02CFB6E2 0000002F
04CFB6D3 0044C460
Seventh lv down
00CFF49B 00000028
04CFF448 00000000
04CFF44C 0005FC61
02CFF4B4 00000183
02CFF4B8 00000133
02CFF4B6 0000005E
04CFF4A7 00457E62
This can be used at any time to bring someone down to level 1, or whatever their initial level is, but it only really needs to be on when you load the new game+ for the first time. Obviously, it should be shut off the first chance you get. In case anybody wants to edit the code for their own use, the first line is the level, the second and third line are exp, but only the third line is taken into account when calculating the level, the fourth line is strength, the fifth line is ether, the sixth line is agility and the last line is hp. There were two problems when using this code. First, the game doesn't automatically adjust anyone's level when they first enter your party, so the level they join at is about the level I was at when they joined. The second problem is that anybody other than Shulk and Reyn have their max affinity coins recalculated when they enter the party for the first time. I'm sure it uses a counter somewhere for how many unique monsters have been killed, but I wasn't able to find it so their coin count needs to be manually edited after they join, or at your leisure, using the following codes.
Dunban coins
02cf2336 00000xxx
Sharla coins
02cf610a 00000xxx
Riki coins
02cf9ede 00000xxx
Melia coins
02cfdcb2 00000xxx
Seventh coins
02d01a86 00000xxx
When I did a playthrough with these codes, I found it was still a little too easy, so I made codes for reducing arts levels and skill trees to their starting values as well. You will still keep whatever arts books and skill trees you had already unlocked.
set all ap to 0
08ce4384 00000000
20083dd4 00000000
Shulk sp point reset
08ce69be 00000000
10050004 00000000
Shulk skill reset
08ce6177 00000000
00040020 00000000
08ce621b 00000000
00050020 00000000
08ce62df 00000000
00050020 00000000
08ce63a3 00000000
00050020 00000000
08CE6467 00000000
00050020 00000000
Shulk skill link remove
08ce652b 00000000
000600c4 00000000
08ce654b 00000000
000600c4 00000000
08ce656b 00000000
000600c4 00000000
08ce658b 00000000
000600c4 00000000
08ce65ab 00000000
000600c4 00000000
Shulk arts level reset
08CE2CE8 00000001
000f0002 00000000
Reyn sp point reset
08cea792 00000000
10050004 00000000
Reyn skill reset
08ce9f4b 00000000
00040020 00000000
08ce9fef 00000000
00050020 00000000
08cea0b3 00000000
00050020 00000000
08cea177 00000000
00050020 00000000
08CEa23b 00000000
00050020 00000000
Reyn skill link remove
08ceA2ff 00000000
000600c4 00000000
08cea31f 00000000
000600c4 00000000
08cea33f 00000000
000600c4 00000000
08cea35f 00000000
000600c4 00000000
08cea37f 00000000
000600c4 00000000
Reyn arts level reset
08ce6abc 00000001
000f0002 00000000
Fiora sp reset
08CEE566 00000000
10050004 00000000
Fiora skill reset
08CEDD1F 00000000
00040020 00000000
08CEDDC3 00000000
00050020 00000000
08CEDE87 00000000
00050020 00000000
08CEDF4B 00000000
00050020 00000000
08CEE00F 00000000
00050020 00000000
Fiora arts level reset
08CEA890 00000001
00040002 00000000
Dunban sp reset
08CF233A 00000000
10050004 00000000
Dunban skill reset
08CF1AF3 00000000
00040020 00000000
08CF1B97 00000000
00050020 00000000
08CF1C5B 00000000
00050020 00000000
08CF1D1F 00000000
00050020 00000000
08CF1DE3 00000000
00050020 00000000
Dunban arts level reset
08CEE664 00000001
000f0002 00000000
Sharla sp reset
08CF610e 00000000
10050004 00000000
Sharla skill reset
08CF58C7 00000000
00040020 00000000
08CF596B 00000000
00050020 00000000
08CF5A2F 00000000
00050020 00000000
08CF5AF3 00000000
00050020 00000000
08CF5BB7 00000000
00050020 00000000
Sharla arts level reset
08CF2438 00000001
000f0002 00000000
Riki sp reset
08CF9EE2 00000000
10050004 00000000
Riki skill reset
08CF969B 00000000
00040020 00000000
08CF973F 00000000
00050020 00000000
08CF9803 00000000
00050020 00000000
08CF98C7 00000000
00050020 00000000
08CF998B 00000000
00050020 00000000
Riki arts level reset
08CF620C 00000001
000f0002 00000000
Melia sp reset
08CFDCB6 00000000
10050004 00000000
Melia skill reset
08CFD46F 00000000
00040020 00000000
08CFD513 00000000
00050020 00000000
08CFD5D7 00000000
00050020 00000000
08CFD69B 00000000
00050020 00000000
08CFD75F 00000000
00050020 00000000
Melia arts level reset
08CF9FE0 00000001
000f0002 00000000
Seventh sp reset
08D01A8A 00000000
10050004 00000000
Seventh skill reset
08D01243 00000000
00040020 00000000
08D012E7 00000000
00050020 00000000
08D013AB 00000000
00050020 00000000
08D0146F 00000000
00050020 00000000
08D01533 00000000
00050020 00000000
Seventh arts level reset
08CFDDB4 00000001
000f0002 00000000
Again, these only need to be on when you first load the new game+ and should be turned off before doing anything significant. The game needs to be saved and reloaded in order for the skill effects to be removed anyway. Linked skills won't be removed just by forgetting the skills, so I made a code to unlink Shulk and Reyn's skills since the game doesn't allow you to do so until a certain point. I would just remove the other characters' linked skills manually as they join the party. It's a little late for this kind of code, but hopefully someone will get as much fun out of it as I have. Sorry about the long lists, I'm sure the codes could be condensed more, but I wouldn't know how.