Can someone explain this trials to me? The only one I kinda understood is the one of reaching Skyhold at level 5 or less... That's seems impossible since you'll most likely be lvl3 after The Wrath of Heaven, and you obtain a lot of xp just by doing The Threat Remains, plus the massive boost of In Hushed Whispers and the battles of In Your Heart Shall Burn are enough to get you easily to lvl 6 at the least even if you keep exploring to the minimum and don't save anyone at skyhold.... In my first Playthorugh I was barely keeping with suggested levels and still reached Skyhold by level 8 with a few ability slots free.
So, Trials... First you check which Trial is required for the achievement, in this case it's called Take it Slow:
Then you go to Options -> Trials, find the Trial you need and toggle the checkbox.
And with the Take it Slow it looks like reaching Skyhold by level 5 might be possible.