Any class can solo retrieve given enough time. I'm guessing your transmuter took a good long while to whittle down an outer guardian.

I'm not sure how you figure it's "not a great problem" for a rank 30 conjurer sans archon.

A thief would have to go through a ton of mercs to kill an outer guardian, since I'm guessing they would last long enough for about 2 circlestabs. If the thief is of the right path to get shield block, uses preps, and has lots of healing gear, it might be possible.

Necromancer has a pet but it dies about as quickly as a thief's merc.

Here's what you do with an offense shifter:

0. Gather large amounts of gold.
1. Gather multiple uses of protection, aura and shield.
2. Put up protection, aura, shield, stoneskin, haste if not already.
3. Attack guardian and stay as long as you can.
4. Run to healer and "heal heal" about four times.
5. Go back to (2) until guardian is dead.

Alternately, get rid of (0) and replace (4) with "Run someplace safe, slow yourself, sleep 3 ticks."

As for PKing...yeah, sucks for shifters, but likewise for certain other classes until they get the right skills/spells/song.

btw, at 46th rank you should have a 4th form.