I have used a shield since starting to play Valheim. Currently I use the Black Metal Shield, the best shield in the game. The trick to successfully parrying an opponent is to wait and raise your shield just as your opponent begins their strike. My first practice was against a troll (thanks to my nephew), and it was so much easier than I imagined. Because a troll's strike is so slow, it is telegraphed, so you can easily see when he begins his swing to strike. 99.9% of the time, whether it is a troll or a fuling, or even a ranged attack, a successful parry will stunlock your opponent, leaving lots of time to generally finish them off, especially if one uses a knife as it is such a fast weapon one can get in multiple hits, enough to kill a stunlocked enemy.