asta dying part kind of implies that it's a necessary part of protecting the kingdom. maybe julian is alluding to the possibility of asta's anti-magic damaging him, which I think would be really lame boring trope(kind of makes it more likely imo cause Black Clover is very tropey). I doubt this one though.
My other thought is that it's actually something very specific like an interaction between magics. Julian hasn't actually seen Asta in action to see any abnormalities, so whatever he deduced would have to come down to something along the lines of lore or some knowledge he'd gained through his exploration of the world and its magics. What if Julian is specifically referring to a single kingdom being a threat, and it's terrain, strongest warrior, or something has magic that's like poison to or counters asta specifically? whatever case, he's meant to lead them to victory again and julian knows it.
