Qwerty
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The Bone Dragon is not weak.
Among all units, it occupies a quite respectable place.
It is weaker than Black, Red, and Green Dragons, as well as Titans.
It can be compared in strength to Giants and Phoenixes, but Bone Dragons are probably slightly stronger.
The weakest level 6 units are Crusaders and Cyclops, and the weakest overall is, of course, the Paladin.
As for Djinns, their strength is quite relative (due to their ability), but Bone Dragons are clearly stronger, as are Phoenixes.