This ach is now fixed according to todays update 8th July.
i will list of the xp you will need from level35 to 45..
I will take a while..
My Playtime: 3 days.. 18 hours (with activated xp boost)
Kills : 52300
35-36 --> 1.100.000
36-37 --> 1.300.000
37-38 --> 1.400.000
38-39 --> 1.500.000
39-40 --> 1.600.000
40-41 --> 2.000.000
41-42 --> 2.500.000
42-43 --> 3.000.000
43-44 --> 4.000.000
44-45 --> 4.000.000