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Question: medal of honor heroes 2 psp level: "port"

When i play the level "Port", i forfill every task that is requested. But when i enter the last door with an arrow pointing downwards(the end), then a secondary task is added to the secondary tasks. something like: "hold position for enemy reinforcements". How can i do this.
This way the level is onely 90% full. So the next level will still be closed.
Please help!