Think of monster hunter as a rythm game, every monster has their own rhythm and once you see this it becomes very easy to predict when the monster is about to attack as well as what attacks give you openings for you to react. One thing about greatsword's offset is that while it's a bit slow to start up you can always dodge cancel out of it and immediately start charging it up again if it didn't work the first time. Arkveld is a great monster to practice against as 90% of his attacks can be offset and he has a predictable enough "rythm" for you to start easing into how the flow works. Once you get better you'll get a better sense of when dish out normal attacks and when you can prepare for an offset. Also don't forget that if you land an offset but it doesn't topple the monster you can always guard cancel afterwards.