You only have very slight turning ability during Unstoppable Onslaught, so make sure to use it in straight paths.
Roar of the Slayer is a great setup ability to land a very powerful Decimating Smash.
The Soul Furnace buff displays how much shield strength is remaining, use this information to time its explosion perfectly.
Even if Sion still hits a Decimating Smash, making him release the charge earlier reduces its impact.
Use the time after Sion has died to reposition and prepare for his return.