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I don't think that Chromatic Orb needs an attack roll, does it?
Nope, although a saving throw to negate the damage and the effect is given (hence it can fail, especially against tougher enemies)
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D4 dmg is irrelevant, but if I understand correctly what you wrote here, I can stop the game when I see one of my enemies started to cast, cast a Larloch on him with my mage. Since Larloch casting speed 1, it will distrupt his spell. Is it works? If yes, it is good spell. Even better than Magic Missile or Chromatic Orb what I use, since CO need an attack roll and Magic Missile is stopped by Shield or magic resistance.
Yes - but I wouldn't get a mage to cast it - they should be casting something like lower resistance (so that you can cast decent spell later in the fight) or buffing etc. A fighter mage would be better suited to it because they could make better use of the rest of the round (ie making decent attacks) whereas a mage cannot. Also your F/M may get some esoteric benefit out of the extra HP while your mage would not (you should never allow your mage to be hit)
Better than Magic missile - probably not. As soon as you bring the enemies MR down below around 70% your missiles will be very likely to hit and they're going to cause a bit more damage than Larlocks minor drain. Having said LMD definitely has its uses.