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Let's say we have an elemental hero with a bolt dealing 20 damage, level 5, with a strength of 7.
The damage per bolt is 7 * 20 = 140. Since the hero can cast the spell 4 times (or even 5, but then they might not have time to escape), the total damage is 140 * 4 = 560, which is enough to defeat 3 Firebirds.
Next, we have an elemental hero with a bolt dealing 20 damage, level 1, with a strength of 3.
The damage per bolt is 3 * 20 = 60. They can cast the spell 5 times, resulting in a total damage of 60 * 5 = 300. This is enough to defeat 2 Firebirds.
Therefore, the first hero should have a strength of at least 8 to kill 4 Firebirds in one battle (or have stronger and more advanced magic).
P.S. But of course, it's hard to imagine a Conflux without good magic.
P.P.S. It's a shame that this method won't work against the green and red you know who hiding in ambush :D