A 1v1 between Servants merely means they use all of their powers, including whole armies they can call, so long as they don't receive aid from other Servants or magi. Unless you're dealing with more honorable sorts, of course.
Gotta keep those summoned monsters like Basmu occupied somehow, since Semi could summon her own monsters at will. Semi is going to have to avoid using dragons, though, since Jalter's Dragon Witch could usurp them.
I'll wager that Semi is powerful enough in Hanging Gardens to compensate for this, but Jalter is in no mood to indulge her and is going to be outright merciless rather than have any sadistic fun with Semi over her son. If Semi isn't throwing all of her best stuff all the time, Jalter will crush her through sheer hate (which empowers Avengers beyond their base means), though Semi is definitely smart enough to know this and will fight accordingly. This could be excessively brutal.
Good odds that the little kid is going to defuse the fight before they get too lethal, though.
You say that, but how do we know every hazard thrown in his general direction won't just conveniently miss? Or rather, getting taken to a safe watching-corner by a stealthy Papa or a certain burger-aficionado?