What you just did seems to be a technique where the user is capable of molding iron sand from a nearby source to his heart’s desire but preferably a tool to be used for offense or defensive maneuvers.
Weakness wise, the formation of the tool being formed by the iron sand may give the opponent time to react.
Although a simple technique, this jutsu can prove to be useful in a battle. With the molding of iron sand, the user can create weapons intended for either offensive or defensive use which is an advantage. Depending on the situation of the battle and the user’s imagination, a technique like this can be used to kill, slow down, or distract the target to set up for something else. Disadvantage wise, the tools created from this technique aren’t that great in size so countering may prove to be easy.
I would personally use this technique to create kunais or shurikens to send at my opponent and quickly move onto the next jutsu or if the field is large enough and there is iron sand in place behind my opponent, I would form either kunais or tiny spikes of iron sand to attack my enemy from there blind spot. Seeming that the weapons created are small, to counter I would use a higher ranked elemental jutsu such as wind to clash with weapons and blow them away and out of harm. Lightning can obviously be used to disrupt the magnetic field before the iron sand is manipulated. A lid or similar technique could be used to drop from above and fall onto the weapons and stop them in their track
Ways to counter…well besides countering the senbons with weapons infused with ration chakra, a high ranked elemental body armor or wall should also protect the target. Submerging/going underground should also keep the target out of harm’s way if time is on their side.