Guardian Ninja may have less hp than armagio, but it can be summoned for free with no turns wasted setting it up. Also consider that the summoning mechanic allows it to be used in any deck regardless of quanta type. It's versatile, and has the unique edge of blocking enemy creatures before they make their first attack, giving you the chance to kill it with a spell before taking any damage, and without the opponent knowing the wall was there until it's too late.