Programming under constraints...

Original :

ld hl, $A420
ld d, $D8
ld a, b
sub [hl]
ld [hl], b
ld hl, $AD69
add [hl]
ld [hli], a
ret nc
inc [hl]
rst $00


Result :

add sp, $9E ; sp = $DF53, PP move 3 of 28th (?) Pokémon
pop hl ; PP moves 1 and 2
pop de ; PP move 3, only d matters
ld sp, hl
pop hl ; ld hl, $A420
push de ; Perform ld a, d
or $50 ; Padding w/ terminator
pop af ; Reminds me of something... hm.
sub [hl]
push af
call LoadToHL ; Increments hl but who cares
or a ; Padding
or $50 ; Padding & terminator
pop af
pop hl ; ld hl, $A420
add [hl]
call LoadToHL+1
di ; Padding
di
ld d, b ; Terminator
call nc, IncAtHL ; Condition inverted by second LoadToHL
rst $00

LoadToHL:
ld a, d
ld [hli], a
ccf ; Used to complement the carry of the second addition
ret

IncAtHL:
inc [hl]
ret