Battle magic works by coalescing the mana inside your body and forcing it out your hands into the spell you wish to conduct. This is physically and mentally demanding, and the user must maintain upmost concentration during the process, or they can damage themselves. This makes this a dangerous for of magic to use.
In recent decades, a workaround has been developed to make the process easier and safer. Tattoos, which are sigil meant to represent the spell being done, are magically formed on the body and only appear during the casting. These sigils absorb the mana into themselves and help the caster coalesce their mana quickly and efficiently. These sigils are permanent, but fade away when not in use.
The drawback is that these tattoos force the flow of mana into a particular "shape", which limits the caster to that particular spell. The caster cannot use any other forms of magic or spells of battle magic until that tattoo is removed. Therefore, it provides quick and safe use while limiting the pool of spells.
This is my issue. If a caster wanted to use a very powerful battle spell, such as a tornado of fire, they could easily tattoo these sigils magically to themselves. This can make them overpowered, even with the limitations of these sigils.
How can I limit casters from easily using these powerful spells?