Did some more matches with Valkyrie. Her Heavy->Shield Bash->Sweep is pathetic. Using a Sweep after a Shield Bash is a complete waste of stamina and pretty much guaranteed to miss against a competent player. Her heavy is slow too and easy to parry or block, so you'll probably get no damage from attempting that combo. On the other hand, a Sweep after a throw or headbutt is guaranteed to hit but does pretty much no damage and requires a Guard Break, which has terrible range.
So far, it seems like her only options are relying on Guard Breaks, which require you to get super close, or Light Attacks, which do little damage and are easily dodged by Assassins. She's pretty much like an Assassin without the mobility and speed. Hopefully I can discover some better tactics tomorrow.