He's insurmountable without serious plot devices as an NPC and as a PC or GM PC he'll just deus ex machina everything, leaving the players feeling cheated out of a decent gaming experience.
You can take him out as a 10 refresh wizard with some prep.
Step 1, figure out a way to survive or avoid the 1st attack. Possibly by ambush from beneath a veil (around power 6 should do the job).
Step 2:
Have a block vs movement that lasts a few exchanges that is resisted by might. Since this guy only has +1 might, a power 4 duration 4 block is probably going to be good enough.
Step 3:
Have an attack spell that is resisted with might, like earthstomp
Combat would look like:
Wizards 1st action: Use block vs movement. This keeps the grim reaper from coming up to you and hitting you, it also prevents him from escaping.
Following actions:
earth stomp a lot. Assuming you are generating control 6 power 6 earth stomps, you are hitting for about 11 shifts of damage each time (9 after armor).
so:
1st stomp: (9 damage after armor, minor consequence + 7 stress)
OOOO(OOXO) minor consequence
2nd stomp:
+2 bonus for tagging the minor consequence
(11 damage after armor, moderate consequence + 7 stress, rolling up to 8th slot)
OOOO(OOXX) minor + moderate consequence
3rd stomp:
+2 bonus for tagging the moderate consequence
(11 damage after armor, severe consequence + 5 stress)
OOOO(XOXX) minor + moderate + severe consequence
[...]
The 4th stomp is good for an extreme consequence, but as your presumed power 4 duration 4 blo0ck is running out, it looks like you won't quite pull it off. But you've come quite close. If you can produce a slightly longer duration block, or pull of slightly more powerful earth stomps, or something, you can probably finish the job.
A mortal with holy touch who knows how to wrestle might also be able to pull this off, since they don't have to eat through the armor and extra stress boxes provided by toughness. Of course, they'd have to avoid the first attack and get a favorable aspect. But a 6 refresh champion of god with great might and some luck might be able to kill you too.