Were You Successful In Casting The Spell?

You just took 9 hit points of damage, and you are casting a 1st level spell. That means that you have to make a COncentration check DC 20. Then you have to beat Spell Resistance 8, which means that you need to roll a 6 on a d20. You do both of those, and then the magic missile will hit.

I already described the fiend's action, and it missed Kaspar. Yes, Kaspar would have gotten in sneak damage.

We have now completed round 4. We begin round 5 with Castille again. This time around a spell will only require beating spell resistance, not a concentration check. Or you could use the spear again.

Then the fiend will go...and I just rolled a 16, with its mods that's another hit. Kaspar will take 6 h.p. of damage.

Assuming Kaspar is still up and attem, he could then attack ending round 5. After round 6 the fiend will disappear.