A Kingdom Hearts VGM challenge
Ruby 134 131/10=13.1
-&gt;g,b{&quot;Rowdy Rumble|Dread of Night|Riku|One Winged Angel|Rage Awakened|Vim and Vigor|The Key|The 13th Struggle&quot;.split(?|)[b.ord%8]}

This is a lambda function, which takes arguments for the game and the boss. The argument for the game is actually ignored by the code, but the OP has clarified that the argument cannot be eliminated.
In fact it only uses the first letter of the boss's name. the ASCII code is taken modulo 8 (this gives the same answer for a given letter, regardless of case) and one of 8 song titles is returned. As two song titles appear twice, this covers a total of 10 elements:
Game   Boss                      Array  Song
                                 index
Coded  Pete                         0   Rowdy Rumble
DDD    Anti Black Coat Nightmare    1   Dread of Night
Coded  Riku                         2   Riku
1      Sephiroth                    3   One Winged Angel
2      Sephiroth                    3   One Winged Angel
2      Lingering Sentiment          4   Rage Awakened
BBS    Terra vs Master Xehanort     4   Rage Awakened
Coded  Maleficent                   5   Vim and Vigor
BBS    Ventus vs Vanitas            6   The Key
CoM    Organization XIII            7   The 13th Struggle

It was a lucky coincidence that the Ascii codes for L and T give the same result modulo 8, so both incidences of Rage Awakened could be covered.
Call the function as in this test program:
f=-&gt;g,b{&quot;Rowdy Rumble|Dread of Night|Riku|One Winged Angel|Rage Awakened|Vim and Vigor|The Key|The 13th Struggle&quot;.split(?|)[b.ord%8]}
B=gets                  #get boss name from stdin
puts f.call(&quot;dummy&quot;,B)  #call function, and output return value to stdout