mass update sailor_moon -sailor_moon_(cosplay) -cosplay -> sailor_moon magical_girl
mass update sailor_mercury -sailor_mercury_(cosplay) -cosplay -> sailor_mercury magical_girl
mass update sailor_venus -sailor_venus_(cosplay) -cosplay -> sailor_venus magical_girl
mass update sailor_mars -sailor_mars_(cosplay) -cosplay -> sailor_mars magical_girl
mass update sailor_jupiter -sailor_jupiter_(cosplay) -cosplay -> sailor_jupiter magical_girl
mass update sailor_saturn -sailor_saturn_(cosplay) -cosplay -> sailor_saturn magical_girl
mass update sailor_uranus -sailor_uranus_(cosplay) -cosplay -> sailor_uranus magical_girl
mass update sailor_neptune -sailor_neptune_(cosplay) -cosplay -> sailor_neptune magical_girl
mass update sailor_pluto -sailor_pluto_(cosplay) -cosplay -> sailor_pluto magical_girl
mass update sailor_chibi_moon -sailor_chibi_moon_(cosplay) -cosplay -> sailor_chibi_moon magical_girl
mass update tuxedo_kamen -tuxedo_kamen_(cosplay) -cosplay -> tuxedo_kamen magical_boy
mass update cure_black -cure_black_(cosplay) -cosplay -> cure_black magical_girl
mass update cure_white -cure_white_(cosplay) -cosplay -> cure_white magical_girl
mass update shiny_luminous -shiny_luminous_(cosplay) -cosplay -> shiny_luminous magical_girl
mass update cure_bloom -cure_bloom_(cosplay) -cosplay -> cure_bloom magical_girl
mass update cure_egret -cure_egret_(cosplay) -cosplay -> cure_egret magical_girl
mass update cure_bright -cure_bright_(cosplay) -cosplay -> cure_bright magical_girl
mass update cure_windy -cure_windy_(cosplay) -cosplay -> cure_windy magical_girl
mass update cure_mint -cure_mint_(cosplay) -cosplay -> cure_mint magical_girl
mass update cure_lemonade -cure_lemonade_(cosplay) -cosplay -> cure_lemonade magical_girl
mass update milky_rose -milky_rose_(cosplay) -cosplay -> milky_rose magical_girl
mass update cure_aqua -cure_aqua_(cosplay) -cosplay -> cure_aqua magical_girl
mass update cure_rouge -cure_rouge_(cosplay) -cosplay -> cure_rouge magical_girl
mass update cure_dream -cure_dream_(cosplay) -cosplay -> cure_dream magical_girl
mass update shining_dream -shining_dream_(cosplay) -cosplay -> shining_dream magical_girl
mass update cure_berry -cure_berry_(cosplay) -cosplay -> cure_berry magical_girl
mass update cure_passion -cure_passion_(cosplay) -cosplay -> cure_passion magical_girl
mass update cure_peach -cure_peach_(cosplay) -cosplay -> cure_peach magical_girl
mass update cure_pine -cure_pine_(cosplay) -cosplay -> cure_pine magical_girl
mass update cure_blossom -cure_blossom_(cosplay) -cosplay -> cure_blossom magical_girl
mass update cure_marine -cure_marine_(cosplay) -cosplay -> cure_marine magical_girl
mass update cure_sunshine -cure_sunshine_(cosplay) -cosplay -> cure_sunshine magical_girl
mass update cure_moonlight -cure_moonlight_(cosplay) -cosplay -> cure_moonlight magical_girl
mass update cure_melody -cure_melody_(cosplay) -cosplay -> cure_melody magical_girl
mass update cure_rhythm -cure_rhythm_(cosplay) -cosplay -> cure_rhythm magical_girl
mass update cure_muse -cure_muse_(cosplay) -cosplay -> cure_muse magical_girl
mass update cure_beat -cure_beat_(cosplay) -cosplay -> cure_beat magical_girl
mass update cure_beauty -cure_beauty_(cosplay) -cosplay -> cure_beauty magical_girl
mass update cure_sunny -cure_sunny_(cosplay) -cosplay -> cure_sunny magical_girl
mass update cure_happy -cure_happy_(cosplay) -cosplay -> cure_happy magical_girl
mass update cure_black -cure_black_(cosplay) -cosplay -> cure_peace magical_girl
mass update cure_march -cure_march_(cosplay) -cosplay -> cure_march magical_girl
mass update cure_echo -cure_echo_(cosplay) -cosplay -> cure_echo magical_girl
mass update cure_heart -cure_heart_(cosplay) -cosplay -> cure_heart magical_girl
mass update cure_diamond -cure_diamond_(cosplay) -cosplay -> cure_diamond magical_girl
mass update cure_sword -cure_sword_(cosplay) -cosplay -> cure_sword magical_girl
mass update cure_ace -cure_ace_(cosplay) -cosplay -> cure_ace magical_girl
mass update cure_rosetta -cure_rosetta_(cosplay) -cosplay -> cure_rosetta magical_girl
mass update unknown_green-haired_cure -unknown_green-haired_cure_(cosplay) -cosplay -> unknown_green-haired_cure magical_girl
mass update cure_fortune -cure_fortune_(cosplay) -cosplay -> cure_fortune magical_girl
mass update cure_honey -cure_honey_(cosplay) -cosplay -> cure_honey magical_girl
mass update cure_princess -cure_princess_(cosplay) -cosplay -> cure_princess magical_girl
mass update cure_art -cure_art_(cosplay) -cosplay -> cure_art magical_girl
mass update cure_continental -cure_continental_(cosplay) -cosplay -> cure_continental magical_girl
mass update cure_gonna -cure_gonna_(cosplay) -cosplay -> cure_gonna magical_girl
mass update cure_katyusha -cure_katyusha_(cosplay) -cosplay -> cure_katyusha magical_girl
mass update cure_nile -cure_nile_(cosplay) -cosplay -> cure_nile magical_girl
mass update cure_pantaloni -cure_pantaloni_(cosplay) -cosplay -> cure_pantaloni magical_girl
mass update cure_sherry -cure_sherry_(cosplay) -cosplay -> cure_sherry magical_girl
mass update cure_southern_cross -cure_southern_cross_(cosplay) -cosplay -> cure_southern_cross magical_girl
mass update cure_sunset -cure_sunset_(cosplay) -cosplay -> cure_sunset magical_girl
mass update cure_wave -cure_wave_(cosplay) -cosplay -> cure_wave magical_girl
mass update blonde-haired_cure -blonde-haired_cure_(cosplay) -cosplay -> blonde-haired_cure magical_girl
mass update green-haired_cure -green-haired_cure_(cosplay) -cosplay -> green-haired_cure magical_girl
mass update grey-haired_cure -grey-haired_cure_(cosplay) -cosplay -> grey-haired_cure magical_girl
mass update orange-haired_cure -orange-haired_cure_(cosplay) -cosplay -> orange-haired_cure magical_girl
mass update red-haired_cure -red-haired_cure_(cosplay) -cosplay -> red-haired_cure magical_girl
mass update unknown_black-haired_cure -unknown_black-haired_cure_(cosplay) -cosplay -> unknown_black-haired_cure magical_girl
mass update unknown_blonde-haired_cure -unknown_blonde-haired_cure_(cosplay) -cosplay -> unknown_blonde-haired_cure magical_girl
mass update unknown_brown-haired_cure -unknown_brown-haired_cure_(cosplay) -cosplay -> unknown_brown-haired_cure magical_girl
mass update unknown_green-haired_cure -unknown_green-haired_cure_(cosplay) -cosplay -> unknown_green-haired_cure magical_girl
mass update cure_scarlet -cure_scarlet_(cosplay) -cosplay -> cure_scarlet magical_girl
mass update cure_twinkle -cure_twinkle_(cosplay) -cosplay -> cure_twinkle magical_girl
mass update cure_flora -cure_flora_(cosplay) -cosplay -> cure_flora magical_girl
mass update cure_mermaid -cure_mermaid_(cosplay) -cosplay -> cure_mermaid magical_girl
mass update cure_miracle -cure_miracle_(cosplay) -cosplay -> cure_miracle magical_girl
mass update cure_felice -cure_felice_(cosplay) -cosplay -> cure_felice magical_girl
mass update cure_magical -cure_magical_(cosplay) -cosplay -> cure_magical magical_girl
mass update mew_ichigo -mew_ichigo_(cosplay) -cosplay -> mew_ichigo magical_girl
mass update mew_lettuce -mew_lettuce_(cosplay) -cosplay -> mew_lettuce magical_girl
mass update mew_mint -mew_mint_(cosplay) -cosplay -> mew_mint magical_girl
mass update mew_pudding -mew_pudding_(cosplay) -cosplay -> mew_pudding magical_girl
mass update mew_zakuro -mew_zakuro_(cosplay) -cosplay -> mew_zakuro magical_girl
mass update tailred -tailred_(cosplay) -cosplay -> tailred magical_girl
mass update tailyellow -tailyellow_(cosplay) -cosplay -> tailyellow magical_girl
mass update tailblue -tailblue_(cosplay) -cosplay -> tailblue magical_girl
mass update angel_peach -angel_peach_(cosplay) -cosplay -> angel_peach magical_girl
mass update angel_salvia -angel_salvia_(cosplay) -cosplay -> angel_salvia magical_girl
mass update angel_daisy -angel_daisy_(cosplay) -cosplay -> angel_daisy magical_girl
mass update angel_lily -angel_lily_(cosplay) -cosplay -> angel_lily magical_girl
mass update amulet_heart -amulet_heart_(cosplay) -cosplay -> amulet_heart magical_girl
mass update amulet_clover -amulet_clover_(cosplay) -cosplay -> amulet_clover magical_girl
mass update amulet_spade -amulet_spade_(cosplay) -cosplay -> amulet_spade magical_girl
mass update hyper_blossom -hyper_blossom_(cosplay) -cosplay -> hyper_blossom magical_girl
mass update hyper_bubbles -hyper_bubbles_(cosplay) -cosplay -> hyper_bubbles magical_girl
mass update hyper_buttercup -hyper_buttercup_(cosplay) -cosplay -> hyper_buttercup magical_girl
mass update prisma_ilya -prisma_ilya_(cosplay) -cosplay -> prisma_ilya magical_girl
mass update phantas-moon -phantas-moon_(cosplay) -cosplay -> phantas-moon magical_girl
mass update saint_tail -saint_tail_(cosplay) -cosplay -> saint_tail magical_girl
mass update meruru_(oreimo) -meruru_(oreimo)_(cosplay) -cosplay -> meruru_(oreimo) magical_girl
mass update star_guardian_lux -star_guardian_lux_(cosplay) -cosplay -> star_guardian_lux magical_girl
mass update pretty_sammy_(character) -pretty_sammy_(character)_(cosplay) -cosplay -> pretty_sammy_(character) magical_girl
mass update ladybug_(character) -ladybug_(character)_(cosplay) -cosplay -> ladybug_(character) magical_girl
Related to topic #13004. The above are all magical girl aliases which should be tagged with magical girl.
Note:This isn't the 100% solution, since there might be pictures of magical girls with other characters cosplaying as magical girls that won't get tagged with this bulk update. Also, this doesn't cover magical girls without magical girl aliases.
I believe I've captured most of the low-hanging fruit with the above list. If there is anyone missing (i.e. magical girls with magical girl aliases), please leave a comment, or edit the request directly (builder+).