start the trainer (don't press F1 yet) log into battle net. Before campaign mission disconnect the internet and you be automatic go offline mode.
That's the only way to enter offline mode? Talk about a pain in the butt. I can't believe Blizzard removed the 'play as guest' option.
Use firewall to block incoming/outgoing for both "SC2.exe" and "Starcraft II.exe". "Starcraft II.exe" being the launcher, its a bit tricky. Unblock "Starcraft II.exe" while the launcher is checking for undates. Before clicking on "Play", block "Starcraft II.exe". It will take longer for the game to load though.
"SC2.exe" can be found in "StarCraft II\Versions", in 1 of the "BaseXXXXX" sub-folders. Always choose the "BaseXXXXX" sub-folder with the highest numbering. The directory of "SC2.exe" will always change with each patch, hence the need to re-configure the firewall settings from time to time.
[Edited by Tingxu, 3/16/2013 9:34:34 AM]
What exactly will this do? Allow use of the trainer without disabling your internet?