change race/class during combat question

[budapest]

If I kick open the door while I'm a specific race/class and the monster I face has special bonuses verses that race/class, can I discard the race/class card and not face the consequences?

For example if I am a dwarf when I kick open the door and get 3,872 orcs can I just say "hey i'm not a dwarf!" and fight them as a human?

Also could I offer to help someone kill a monster and during combat play my elf card so I would go up a level when that monster was killed?

Thank in advance!