I've increased Cybella Gunslinger PVE rating to S. Her 4CD sleep for 2 turns along with the 100% multiplier trigger finally convinced me to give her that S rating.While you guys have many valid points, I really still do not believe GS Cybella is same tier as Talisa.In the current available PvE content, I will always bring trigger rangers over any mystic. Ronan should be downgraded to C, and Talisa to B. Or GS Cybella should really be S-tier. A single trigger proc from a ranger can easily deal in a single turn (by proccing Sven and other rangers to attack) much more damage that all the damage Talisa and her pet can do over 6 turns (they both use their skill), it's not even a comparison.
There seems to be a heavy reluctance to give non-summoners S-tier in PvE, that there are only 4 units rated S.
The 4 turn cooldown on Talisa is a big deal. It is just way too slow, compared to the damage output trigger rangers can dish out by triggering adjacent heroes (champs and other rangers) in the first 3 turns that can outright kill half the enemies while Talisa just tickles them. This effectively reduces the damage my squad takes significantly, so that your squad requires less healing/tanks/potions. If you let the enemies live so that Talisa can mass damage them, yes that's a lot of damage, but the fight just goes so much slower. If I can kill most of the enemies in the first 2 turns, I don't need Talisa's pet to distract. Ranger triggers are so important in PvE that most of the time, I would rather normal-attack with my rangers and never use their skills except in an emergency situation where I need to stun/sleep a enemy unit. This is the part where I address the "hero effectiveness vs enemies" - killing enemies is just more effective than dragging out the fight for Mystic skills to kill them - especially a skill that deals damage over time and not instant damage to kill.
There is no reluctance in giving non-summoners S-tier PvE, it's just that an extra unit is such an advantage in PvE content and Summoners are the only units that increase your MAX unit count with units that are useful. Not to forget the icing on the cake which is the summon skill effects that most summoners have.
Yes, killing enemies is more effective than dragging out the fight, providing you can deal significant damage. As mentioned earlier, rangers tend to have difficulty wiping high DEF enemies which are much more common in PvE than high MAG enemies. More so if the ranger is CRIT built since it means sacrificing ATK. Another thing to consider is that trigger teams are RNG based while Talissa is less risky.
MW Ronan is a "B" in PvE because his DEF reducing aura and CD delay skill provides good utility in PvE.