-When you have both goals requiring the same Boss Level at the same time, the kills count toward both.
-You can start multiple bosses of different levels, at the same time.
-So, depending on the order of Boss Level for each goal, it may be possible to do multiple bosses and not waste your ability to kill two birds with one stone (completing a Normal and Elite goal with the same kills)
Easy:
18: Lv50 1x
19: Lv45 5x
20: Lv50 5x
Normal:
8: Lv50 1x
9: Lv45 4x
10: Lv55 1x
11: Lv50 5x
12: Lv60 1x
13: Lv55 5x
14: Lv65 1x
15: Lv60 5x
16: Lv70 1x
17: Lv65 6x
18: Lv75 1x
19: Lv70 6x
20: Lv80 1x
21: Lv75 6x
22: Lv80 7x
Elite:
1: Lv60 1x
2: Lv65 1x
3: Lv60 5x
4: Lv70 1x
5: Lv60 3x
6: Lv65 6x
7: Lv75 1x
8: Lv65 3x
9: Lv70 7x
10: Lv80 1x
11: Lv70 3x
12: Lv75 8x
13: Lv85 1x
14: Lv75 3x
15: Lv80 9x
16: Lv90 1x
17: Lv80 4x
18: Lv85 10x
19: Lv95 1x
20: Lv85 4x in a row
21: Lv 90 11x
22: Lv 100 1x
23: Lv 90 5x
24: Lv95 12x
25: Lv95 7x in a row
26: Lv100 15x
27: Lv100 10x in a row