Boss monsters are not reflected in any way in the kill list or achievements. It would be awesome to have them somehow distinguished from the rest monsters.

Two possible solutions:
a) the kill list at the end of the game shows (...of which *foo* were bosses);
b) achievement string ("(S)He had an opportunity to face *foo* bosses, of which *bar* escaped their destination.");

Extra awesomeness for listing the names of killed bosses:
a) ...of which *foo* were bosses (Name, Other name);
b) achievement string followed by the list: Name, the kind of monster (killed / escaped).

Even more extra awesomeness for listing where a particular boss was first met, although that might be heavy to implement.