Ah, if you hardcoded options in HTML, it will be very difficult to implement this. Instead, put all the monsters into array, and then present the <select> with a foreach loop:
$monsters= array ('Baby Elf', 'Wood Elf', 'Dark Elf', ...);
And then:
...
$monster_selected= (isset($_POST['monster_selected'])) ? ((int) $_POST['monster_selected']) : 0;
...
echo '<select name="monster_selected">';
foreach ($monsters as $key => $val) {
if ($monster_selected==$key) $sel='selected ';
else $sel= '';
echo "<option {$sel}value=\"{$key}\">{$val}</option>";
}
echo '</select>';
It would be wise to keep key=0 empty (for instance "-- Select Monster --") to check if user actually selcted any monster, or did not touch the drop down at all.