I tested the strengths of several shields in the Advanced Training Hall over spikes and fire grates. Both deal normal damage here. These are the shields I used:
Aegis, lvl 10, 8.5 bars normal def, 9 bars health
Ancient Plate, lvl 7, 10+ bars normal, 9 bars health
Volcanic Plate, lvl 10, 10+ bars normal, 9 bars health
Stone Tortoise, lvl 1, 6.5 bars normal, 10+ bars health
These are the results:
Aegis: spikes, broke in 7 hits; fire, broke in 4 hits
Ancient Plate: spikes, broke in 10 hits; fire, broke in 5 hits
Volcanic Plate: spikes, broke in 9 hits; fire, never broke
Stone Tortoise: spikes, broke in 7 hits; fire, broke in 5 hits
Bonus: CoA & GOS broke over fire grates in 7 hits
What's significant about these results is that, although the Stone Tortoise had a good 2 bars less defense than the Aegis, it broke in about the same time, and didn't break as easily over fire. Also note that Ancient Plate has more normal defense than Volcanic Plate, as evidenced by the fact that it took longer to break over spikes than did Volcanic Plate.
Any thoughts?
Because you, unless it is a typo, used a four star Stone Tortoise instead of an Omega Shell.