ɝ and ɚ are the same thing.
To make the sound, just say ə and then the ɹ. You might find Wikipedia's audio sample helpful:
R-colored vowel - Wikipedia, the free encyclopedia
As for the /tn/ in mountain, you might find this video helpful:
YouTube - ‪Lesson 14 - Glottal Stop - English Pronunciation‬‏
- For Teachers