Advice for +Atk -Spe Julia?

I have one with exactly those IVs and shes still very good. After running the battle sim a lot these not really much that gonna change based on her Spd (unless you have the +Spd boon then you can try a few different setups). Since she already hits like a truck i would throw a breaker skill on her (G Tomebreaker is usually more popular but its really up to you) and watch her her to work. Her high res means she can duel most magic opponents just fine. The only thing you'll have to watch out for are those pesky archers since she doesnt have very good Def and tends to get doubled a lot but otherwise shes still a very solid unit.

I have the same one as you friend, I run Death blow 3 and Bowbreaker since her low speed makes her prone to being doubled by archers now, but because of every archer possessing a Brave Bow+ you'll die if they initiate on you anyway so going with G Tomebreaker might benefit you better. Death Blow 3 I'd prefer to use since she's a bit too slow now with those IV spreads but if you happen to grab +spd in the future you'll want fury 3.