The Sandworm

a ravenous creature that move underground(picture: a snake like thing with enchinoderms and large jaw with big tooth). This troop is invulnerable whenever it is moving(underground; which means only receive damage when attacking) and can go across under walls(maybe even deal some damage to it.....).Its favorite target is defenses. It deals melee single-target damage.
Data(I'm not really sure):
lvl1;housing 18??;training 30min??;mv speed 10;cost300DK;atk speed 2s;
DPS135(+-15);DPA270(+-30);HP425(+-50);

And....It's just an idea and I didn't do much calculations: improvement welcomed

Moles have been mentioned, this is very similar.

The idea's OK, but ppl with lvl 11 walls and maxed air defences won't like it ;)