Concentration Checks wrote:Well... it doesn't say 'attack' it just says 'damage'. So you would add all of the damage together before determining the DC of the check. So add up all the damage from the different acid arrows to total up the check; same as with magic missile.
If you take damage while trying to cast a spell, you must make a concentration check with a DC equal to 10 + the damage taken + the level of the spell you're casting. If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between the time you started and the time you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).
If you are taking continuous damage, such as from an acid arrow or by standing in a lake of lava, half the damage is considered to take place while you are casting a spell. You must make a concentration check with a DC equal to 10 + 1/2 the damage that the continuous source last dealt + the level of the spell you're casting. If the last damage dealt was the last damage that the effect could deal, then the damage is over and does not distract you.
Even if it's reasonable to adjudicate that you total Magic Missiles (I don't think it is), the continuous paragraph indicates "source", singular, so it's lots of individual checks.
I think it's individual checks per damage roll anyway, which is much more favorable to the caster and likely intended.