Tsundere is pronounced tsoon-deh-reh.
Sounds in japanese are very consistant. Most of the time, a letter (in transcriptions) will always give the same sound.
A is always pronounced as the A in "cat"
E is always pronounced as the E in "bed"
I is always pronounced as the EA in "eat"
O is always pronounced as the O in "orange"
U is (almost) always pronounced as the OO in "food" (but sometimes it is not pronounced at all)
(note: some groups of vowels are used to write long vowels: AU is long A, EI is long E, II is long I, OU is long O, UU is long U)