It's a more complicated situation because of the Immobilize, but from what you say it seems you're doing several things wrong.
First, determine who wins the fight. In a normal combat, you roll your attack dice, your opponent rolls his attack dice. Highest roll wins the fight, if it's a tie, the model with the highest Fight score wins. If it's still a tie, roll 1 die, 1-3 you win, 4-6 your opponent wins.
Second, whoever won the fight rolls to wound. Normally, they roll the
same number of dice they rolled to win the fight. (There are a few exceptions, such as being trapped, knocked down by cavalry, etc.)
Example from your setup: Boromir rolls 3 dice, the Uruks roll 10 dice. If Boromir gets a 6, he wins automatically, because he has a higher Fight score than the Uruks and the best they can get is a 6. If Boromir wins, he rolls 3 dice to wound, potentially killing 3 Uruks. If Boromir loses, if he has Might left he can use it to match the highest die roll from the Uruks (which he wins, because he has a higher Fight).
Immobilize changes things: Boromir only gets to roll 1 die, and his Fight score is a 1 for this fight. Since the Uruks now beat his Fight score, all they need to do is tie his 1 die roll. If Boromir wins (not likely, but stranger things have happened), he can not roll to wound.
Example 2: say Boromir is not Immobilize. With 10 Uruks, he's probably surrounded, and therefore trapped. Say Boromir rolls 1, 2, 4 and he has only 1 point of Might left. Unfortunately one of the Uruks rolls a 6, which means Boromir loses the fight. Normally the Uruks would then roll their 10 wound dice, but because Boromir is trapped, each attack has a double chance to wound, so they actually get to roll 20 dice. Dead Boromir
Hope that helps.