Deathstroke's age in the comics is a bit fluid. It often depends on the creative direction of the story. Usually, he's thought to be in his mid-40s to early 50s.
First, you need to understand Deathstroke's character. Know his motives, his skills, and his past. This will help you write him in a way that is true to the original. For example, his mercenary nature and his family issues are important aspects to consider.
First, create a unique and believable OC. Give them a distinct personality, backstory, and motivation. For example, if your OC is a former assassin, explain what led them to that path. Second, understand Deathstroke's character. His skills, his code, and his relationships. Then, build a plot that brings your OC and Deathstroke together in an interesting way, like a common enemy or a shared goal.
First, you need to understand the characters well. Naruto is known for his positive attitude, his strong will, and his powerful ninja skills. Deathstroke is a highly skilled assassin with a complex personality. Then, create a unique plot that combines elements from both universes. For example, you could have a situation where the Sharingan (from Naruto) has a strange interaction with Deathstroke's enhanced physical abilities. Also, don't forget about good dialogue. Make it so that the two characters can have interesting conversations that show their differences and similarities.
Kurama might be the link that brings Naruto into Deathstroke's orbit. Since Kurama is such a powerful and unique entity in Naruto's world, his presence could be what makes Naruto aware of Deathstroke in the first place. For example, Kurama could sense some strange energy similar to Deathstroke's and draw Naruto's attention to it.
To create a believable relationship, start with their fundamental differences. Robin is a hero, and Deathstroke is a villain. They could be initially at odds, constantly trying to outwit each other. But then, introduce some common ground, like a shared enemy or a mutual respect for each other's combat skills. This can slowly build a complex relationship.
Naruto's chakra - based powers, like his Rasengan, could be combined with Deathstroke's strategic and combat skills. For example, Deathstroke could use his knowledge of combat tactics to find the perfect moment for Naruto to unleash a powerful Rasengan, maximizing its impact.
Deathstroke has made a significant number of appearances in comics over the years. It's hard to give an exact count, but he's been a recurring villain in many storylines.