Sub tank helps quite a lot when it comes to boss fighting.You do not want to waste time fighting all over again through the same area when X or Zero dies off.You mut know the right time to use the sub tanks and mut be conservative.
SUB TANK.
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This E tank can be found in the Magma Area.You must go through a portal to get to the second area.It is kinda easy to be found because you will bump into it on the way up.
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For the second E tank which is located in the Amazon Area, you must have the blade armour so that you will be able to dash upwards.
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The W tank is not that important but you might need it to complete your collection of tanks.When you go through the laser institute, try go through a portal and you will find it just at the beginning of the second area.
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I don't really know what this tank is called but you must defeat blaze heatnix before getting to the tank.