Subtank
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This is the wire sponge area and the e-tank is just above the area.Get to the moving panel then jump to the left side of the screen.Kick up the wall then jump to the right side to grab the wall.Do not let X fall down or no you will have to start all over again.
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This is also one of the easiest hidden area to find.In Flame Stag's stage, you will see a hovering beetle.Jump onto the beetle then let the thing hover you all the way up then jump to the left of the screen.
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This e-tank can be reach easily without any weapon or upgrade.Your timing must be good.Jump on to the moving blocks then let it bring you closer to the pit.Before it gets into the pit, jump as far as you can to reach the wall.
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To get to this e-tank, charge up the bubble splash then you will be able to jump higher.Cling to the wall then jump on the water surface.Jump until you are able to get to the heart tank.