Try to get Rylai's Crystal Scepter early becouse you get bonus HP good AP and u can slow your enemies.After that get Mejai's Soulstealer and try to stack it to 20.After that get Rabadon's Deathcap and u will get 110 AP plus 30% its passive of that item.And at the edn get 2 Hextech Gun Blades so u get bonus AP AD and Spell Vamp and Life steal

When You are Katarina u can use your shumpo to escape from your enemies.Do it on your team champions or on minnions.Also when u kill an enemy your CDS are reduced by 15 sec.And try to spam Bouncing Blade as much as u can.

I take Ghost and Ignite for summoner's spells becouse with Ghost i get bonus MS so easier to esape or move faster to your target.And ignite to kill low hp Champions

Ok any 1 who wants to add me he can do it :).Im palying on US region and named as Dragangradjan