Three hour paint vomit from last night.
Master Daubenmire takes one for the team. Merely a hypothetical situation! Spawned by a mind that was listening to too many sad songs too late at night. I like to think Morgan'd be smart enough to become something armored before placing his vital organs between an arrow and his best friend. Sometimes our hearts move faster than our heads.
He probably fantasizes about doing this kind of thing, so he can spend a few weeks in bed being fussed over and adored.
This seems a fine place to drop a note on Morgan's mortality, as some people've rightly expressed concerns over Morgan's god-like shapeshifting making him too powerful. Let us play a round of Kill Morgan, a game he loves to hate me for. C:
First, we'll show his ace. Morgan's unshifted 'true' form is an organless, bloodless, generally impossible bit of slime. It 'lives' because he is a corrupted degenerate being of black magic, nuff said. (Morgan is arguably undead, but another story.) Normally if Morgan is injured his injuries carry between shapes, but since this thing has so little resemblance to anything biological taking this form 'clears' wounds.
The price? If he does use this shape while injured he must stay in it for hours at the least, typically days. His bodily injuries translate into general agony and crippling weakness.
The bad thing about slime Morgan (for him) is that he's about as strong as a wet noodle, near-sighted, has no sense of smell or taste, and is pretty fugly. On the other hand, it can be a fairly resilient form. By merit of having no conventional insides, he can survive being stabbed and shot and squashed and can't be suffocated. You can kill him with fire, freezing, by splattering him across enough of a distance, possibly poison, certainly starvation, and with some breeds of offensive magic.
So if the shapeshifter sustains any kind of deadly injury- if he has presence of mind and happens to be conscious- he can try to save himself by unshifting. Doing so means he's immediately helpless and at the mercy of his friends and enemies. Not an ideal back-up plan. If Morgan is for some reason knocked out, paralyzed, or in shock, Magda is able to force him into unshifted form... if she's around. Otherwise, he's toooast.
If you wanted to dispatch Morgan, any manner of sudden death will do the trick. Any snipe attack, broken neck, broken back, axe between the eyes, giant falling boulder, fast-acting toxin, trodding his little mouse body flat, etc. Morgan is resourceful, quick, unpredictable, capable of becoming immensely powerful within seconds and not particularly stupid, so while a strong team of fighters could hold him off for a while and maybe wound him enough to make him flee, it's foolish to take him in a straight, unrigged fight. Because it won't be very straight or even very much of a fight. But just a tiny bit of bad luck on his part would end him. Giant dragons respond similarly as men to a bullet to the brain. And are much bigger targets.
(Ah, and yes, travesty of travesties, the gun has been invented in this high fantasy world. They've not got them down to an art by any means but they're fully capable of blowing the life out of enemies, if you can get them to connect. Many people still prefer crossbows when accuracy counts. With things like Morgan out there, I had to give them firearms. It was only fair.)