I look my first shot at Winterspring this AM and got crushed out the gate. I don’t have a full compliment of 30s yet, but one is 27 and one 28.
Rather than detailing who I got, what has worked for you? Any mercs especially good for the zone? Good synergies for it?
TIA
I totally cheesed Ahune by finding that Treasure that kills summoned minions.
This may bounce around some, as I’m barely into my first cup of coffee this morning and just adding information as I recall it.
Cariel, Xyrella and Samuro. Use Cariels skills to keep her taunting and alive for drawing aggro. Xyrella skill 1 + Samuro skill 1 = kill all. Pop Sams immune aoe when you need to apply extra heals to Cariel with Xyrella. Fire is a good as backup (Ragnaros, Tavish, Antonidas etc). If not, Nature also pairs up well for Winterspring depending on availability. Ragnaros’ Sulfuras equipment can help with the freeze tactics some use.
It will be a lot of Owlkin and other creatures that use a lot of “Attack at end of turn” passives, so a good meat shield is a must. There are also a lot of creatures that apply Windfury to everyone, so you need to time for those to maximize your survival/output.
Ties resolve on cast order, so I generally will order mine left to right for simplicity.
I would recommend Firestone deck tracker thru Overwolf. Useful UI additions to be able to track and monitor skill CDs of allies and enemies. Has task tracking and helpful highlights for treasures received after each fight.
Watch the enemies. I’ve noticed many getting “stuck” in a skill and repeating it over and over without doing anything else. Not super common, but does happen. There seem to be a few bugged skills that pop up on both sides in Wintergrasp (i.e. Taunt minions randomly losing multi-turn taunt buffs).
Cairne, Malfurion, Guff and/or Bru’kan can also progress things for ya, but they are a little slower offensively until you get the ball rolling. Their Nature abilities create some strong defensive/survival combos which easily make up where they lack in offense.
Does Hot Potato make us all cringe when it occupies a Mystery node? Yes. Should I avoid it if I get it? No. Great way to boom in some extra damage, clear trash and potentially soak some of the damage for your team. Much more useful than most make it out to be. Don’t shy away from using it as another tool in your kit when it shows up. I recall one final boss that kept summoning minions, so I wiped them with the tater. Can also be a good target to get progressive buffs rolling when you don’t want to hit the main target on a turn (i.e. it would kill your merc to attack the main this round).
As for equipment, I’m not really sure I know enough to say for sure, but I like a few things for my main 3. These are only my personal views and may/may not be the consensus within the community.
Cariel: +dmg when taunting or heal all on skill 1 deathblow
Xyrella: Illum buff or Flash heal
Samuro: Sash(?) for 2xMirror Image or +attack to 1st skill.
Hoping others chime in here and either add to, or correct, whatever I have said here. I will add to my reply here as I potentially think of anything else that could be useful.
As for online info, I’ve mainly been combing thru reddit and going from there outward.
r/HSMercenaries
edit: I feel like 30 is a must for most Mercs in WS. I think it involves equipment on some.
edit #2: Beast Cleave feels strong as long as you are careful. Dragons and Humans have not been good for me.
Almost TMI, Memnarch! 
(brain esplodes) So much good info to dig through.
Thanks especially for the Firestone app suggestion! Who knew?
Glad I could assist.
The Firestone app has been a huge help for Mercenaries. They were right on top of getting it updated for the new mode. I could sit here and do a whole writeup on Firestone and the tools it provides, but it does a good job of explaining itself. Overwolf does a good job of bringing together a lot of apps like this into one central hub.
I would add that, while not a must, some Mercs just can’t handle Winterspring+ before 30. The spastic nature of the power creep on some of them is insane.
Also, there are some good guides out there that show the best skills and how much you should level some of them. Some Mercs perform worse when certain skills are max level (i.e. Anything that makes Samuro’s skill speeds faster without increasing the speed of the entire team hurts his damage output a lot). Otherwise, you end up like me and blow resources on everything, regretting it later lol.