Card Rulings:Serial Spell

You cannot level up "Horus the Black Flame Dragon" from 4 to 8 with just "Level Up!" and "Serial Spell". Because you would send "LV4" to the Graveyard for "Level Up!", but even if you copy it with "Serial Spell", the first "Level Up!" hasn't resolved, so "LV6" isn't on the field to send to the Graveyard as a cost for the copy of "Level Up!" If you have "Horus the Black Flame Dragon LV4" and "Armed Dragon LV3" on the field, and send one of them to the Graveyard with "Level Up!", you cannot use "Serial Spell" to Level Up the other monster, since there was no monster sent to the Graveyard as a cost for "Serial Spell", and "Serial Spell" would have no point of reference to resolve its effect.