Yeah, there's an overkill with the abilities; this monster will be killed before it can fully realize its potential, and yes, the stat increase is too high, so I wouldn't even bother trying to defeat such a monster. There are too many of them, and they are too strong.
Now, about what made me frown – immunity to fire and water. These are opposing elements, and one cannot counter the other. So, fire won't deal damage to water, but lightning and stone spikes will work normally! You could even explain this in the description – for example, the Chaos faction created a super mega turbo water elemental that is immune to fire to disrupt the balance of the spirit world... not bad, you could even create a map with a final boss, Neptune's warrior, based on that description.

Added 6 minutes later
And the stats are huge: 40 defense + 200 HP + armor + protection from arrows + magic resistance + infinite retaliation + demonic sword = it can wipe out an entire army in one turn. Only a Cave Lord can kill them, but he won't be able to reach them in time.

Added 1 minute later
You could make it a level 8 creature, or give it a 0 stat increase – then there will always be only one of these creatures in the stack