Best option is a train from Somma Lombardo (the town where the HGI is located) to Milan. From HGI to the station a short 10-15 mins walk.
Travel time 65 mins. Cost 5.5 €s.
Unfortunately no direct service to Central station, so you have to change at Rho or otherwise plan a 3 minutes metro ride from Porta Garibaldi station to Central station. trenitalia.com for info.
If you feel unconfortable with this option take the shuttle to the Airport.
From MXP bus is cheaper than the train so I can suggest this option. Remember 15th Aug is an holyday in Italy so Milan will be almost deserted. The same for the Hilton being a business hotel, so you could reasonably expect a nice upgrade.