Well you have to remember, it loses 1 attack each turn and the last attack is only for 2 and often it will be blocked by a shield so this doesn't do as much damage as it appears.
6+5+4+3+2=20
Toss in a shield and 5+4+3+2+1=15
Toss in an upped shield: 4+3+2+1+0=10
Not to mention any time it has a chance to miss. Even without shields or anything it's on par with an Immortal. Over the course of 5 turns, each does 20 damage, and this costs 1 more
.