Maybe like this?
11th - Additional favored shape, +1 level of existing arcane spellcasting class
12th - Battle mastery +8, +1 level of existing arcane spellcasting class
13th - Additional favored shape
14th - Bonus feat, +1 level of existing arcane spellcasting class
15th - Additional favored shape, Battle mastery +10, +1 level of existing arcane spellcasting class
16th -
17th - Additional favored shape, +1 level of existing arcane spellcasting class
18th - Battle mastery +12, bonus feat, +1 level of existing arcane spellcasting class
19th - Additional favored shape
20th - +1 level of existing arcane spellcasting class
Bye
Thanee