When you to the ramp room, immediately conc the defense,
then take out the sentries.

How do you conc them without them spotting you?
Here's how:
The defenders are usually standing at the end of
each ramp out of view.  You don't have to launch
concs onto the upper floor to conc them, just
throw them on the floor of the main level. 
One under each ramp exit. When they explode,
they will conc the guys on the upper level.
Take out the sentry while they are in a daze
and go for the flag.

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