* Aber viel Glück dabei! This is a fairly direct translation and generally works well. "Aber" means "but," "viel Glück" means "good luck," and "dabei" means "with it" or "in doing so." This is polite and somewhat encouraging, even if skeptical.
* Aber viel Erfolg! This is similar, replacing "Glück" (luck) with "Erfolg" (success). This focuses more on the outcome than just chance.
* Na dann viel Glück! "Na dann" adds a slightly more casual and slightly dismissive tone. It's like saying "Well then, good luck!"
* Viel Glück damit! This is similar to "Aber viel Glück dabei!", but slightly more concise. "Damit" also means "with it" or "in doing so."
* (Ironisch) Viel Erfolg! Adding "(Ironisch)" (ironically) in parentheses makes it clear you're being sarcastic. This would be appropriate if you think their endeavor is highly unlikely to succeed.
The best option depends on your tone and relationship with the person. If you're unsure, "Aber viel Glück dabei!" is a safe and generally appropriate choice.