You select the monster you want to destroy when you activate the effect of “Armed Dragon LV5”. This effect targets.
You select a monster for the effect of “Armed Dragon LV5” using the monster’s current ATK, not the original ATK.
You cannot activate “Armed Dragon LV5”’s effect if there isn’t a monster you can destroy.
If a level monster fulfills its condition to Special Summon the next-highest level (for example, if “Armed Dragon LV5” destroys a monster as a result of battle), and the level monster is then flipped face-down, or removed from play permanently, or sent to the Graveyard and then Special Summoned from the Graveyard, then its condition is reset and it is no longer considered to have met the condition to level up to the next level.