I had more luck blocking him with my sword than with my body. It also helps to summon him with the horn while facing north so that he faces north when he enters the room. Soldiers are obnoxiously fiddly and impossible to predict, and playing through this room a few times again I found that altering my position by just one tile in the wrong direction makes it so that he behaves completely differently, so here's a complete description of what I did:
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×Summon the soldier while facing north. Then quickly move to tile (18,6) while facing northwest.
Also, one of your assumptions about this room is a little bit off.
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×The soldier only needs to stab one rock giant.
You can kill him immediately after (I did at least).
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×Remember that killing golems on shallow water makes stepping stones which can be used to stab subsequent giants.
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Links to neat forum tools that I always have trouble finding:
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