2atk, 4atk, 6atk, 8atk, 10atk, altogether, 30 damage in 5 turns for 3
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.
3atk, 6atk, 9atk, 12atk, 15atk, altogether 45 damage in 5 turns for 3
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.
That's an average of 6|9 damage in those 5 turns for only 3
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. I feel it needs at least +1 or +2 to summoning cost.