際に/際/際は => occasion, time N2
辞書形 => Dictionary form N5
Conjugate verbs
[際「さい」]して/に([際「さい」]し)/に[際「さい」]しての => At the time of A (which starts now).. N2
Time, Situation
(の中)で~が一番~です => out of this group, [A] is best N5
に際して => on the occasion of; at the time of ~ N2