You need a magnitude 2 hold to affect a minion, a mag 3 hold to affect a lieutenant and a mag 4 hold to affect a boss. Every 2 seconds, Choking Cloud/Entangling Aura provides a 50% chance of a mag 2 hold and an 80% chance of a mag 1 hold. After 2-3 pulses, these should stack to mag 3 on everything in the AoE. Char is a mag 3 ST hold available in the Fire Mastery Epic (there are other ST holds available in Elec, Leviathan, Mace, Mu, Psi and Soul Mastery). When Char is used on a boss within Choking Cloud's AoE, the magnitude will stack to mag 6 and the boss will be held.