This special phrase can be roughly translated as ‘‘Victory/Success with one-shot’ or ‘One hit, certain kill/death’.
Many Japanese find this phrase very exhilarating. You may also find this in almost every manga, comic or anime. It consists of the following characters:
一(ichi)=one or single
撃=shot/ hit /blow
Therefore…
一撃(ichi-geki)
It literally means single shot/hit/blow
必(hitsu)=100%
殺(satsu)=killing
Therefore
必殺 (hissatsu)= It implies ‘absolute kill’; or ‘certain kill’.
This phrase can be used in a wide variety of context because it means to ‘complete’ or ‘achieve’ something with one action. These Kanji were used in traditional karate because karateka (a karate practitioner) was taught to attack multiple opponents simultaneously.
If you are interested in these Kanji, you may simply copy them from this website and use them as you please. Make them yours!