A Horned Frog starts 3|3. Converting 3 attack to HP causes 0|3+3 = 0|6. Converting twice the attack into HP causes 0|3+(3*2) = 0|9.
The reason I've started by limiting the targets to friendly creatures is that 2
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to flatten an enemy Dragon regardless of its HP is a bit much, I think. You would always want to use this against your opponent's highest-attack target, as it does absolutely nothing to neutralize enemy creatures' abilities.