To be short: she’s just OP.
Just look at the top 100 rankings - nearly everyone has Guinevere as their front hero. There’s no diversity there if you have Guinevere you place her in the front.
If this game is meant to have an invincible set of heroes then it’s ok, but I think that balancing the game to prevent OP combos is in everyone’s interest.
What I propose is to make her mana gain slow or even very slow, or make her drain less mana from enemies.
GUIN ‘THE TANK’… Yes she is absolutely the undisputed master of the middle. Those fortunate enough to have her are at a great advantage to those that don’t (and that is so many of us). Just the way they combo’d her, it turned out to work amazingly well. While I’m not a fan of nerfing I wouldn’t be disappointed if they did something such as you suggested @cicatrix.
Now if I had her, I might not be so agreeable ha ha - and I’ll get to another option in just a moment. I’m about the playing field being a bit more fair. As I read over and over in the forum no matter how many times I try to summon her when Knights makes its round once a quarter it still just comes down to chance and a very very minuscule one. Saving, preparing, waiting or even spending lots of cash doesn’t actually help seems to be the census. If you happen to be a decent and dedicated E&P player, it sucks that someone just by absolute luck gets to smash ya! Ha ha. Proof of her power is very clear as you have stated - Top 100 - she is in just about anyone’s team that is a regular on the leader-board (and I follow this data very closely).
In the end - I love her! She is amazing! Nice job fellas!
SGG truly ‘Balancing’ some of the existing tanks would be a great Idea and my suggestion!
Razor
… and everyone of them would be furious if they nerf her that much. Btw: it doesn’t matter much how much mana she drains. The annoying thing is that you can’t fire your special any more, no matter if 20% is missing or less.
There is some kind of counter hero: panther. But also hard (=expensive) to get.
I don’t have her, but I’m against nerfing. Especially the event 5* are so hard to get, most people probably spent a fortune to get her. Maybe a little nerf is ok, but not something like changing her to slow mana. That’s too much…
They should get sartana a double damage against yellow. This simple change would bury guin.
If not nerfing, give people more ways to beat her - buff other heroes to do more damage to yellow or smth.
And it should be more than one.
Guin is useless for farming, bad for Titans, bad for raid attack and bad for AW attack. She is the best defensive tank for raids and wars, yes, but is still eminently beatable.
Not OP, I think.
(I don’t have her, btw.)
Owl is bad in all of these and is a bad tank also. We need diversity.
People who have her paid enough for it, and it is not fair to nerf her. But it is also not fair tohave such an unique hero. What is needed are heroes that can face her in equal terms
Ps - Imdo not have her
Yeah it would! However, Sartana is already a bad ■■■ as it stands - one of the best (currently). She doesn’t need any more power either! Yet good to know that some of the heroes outside of the special or limited offerings are in this class.
Yes when you do actually pay large to land a limited hero - them getting nerfed is very hard to be ok with! I was completely unhappy when they nerfed Athena, I got over it. And when they reintroduced Hel… let me go spend some time in the corner of my closet (i’ll be right back)…
This. It appears that if you want to be in the top 100 you must have Guinn. This hardly seems fair. She’s not even a regular hero.
Ok, we can make domitia has double damage against yellow and don’t have the bonus defense instead. She will be a good hero after this.
Yeah unfortunately fairness and skill isn’t a huge part of the game - I live with it - still really enjoy it. It’s kinda like life…
Guinevere is also surrounded by a very strong flank and rear set of heroes in that setting - it’s not her alone that gets you on the leader board at all - that’s why in the millions that play this game, the regulars in the Top-100 are up there because of the team of 5 (and whatever tips/tricks they have learned in raiding to get a high trophy count and most importantly their dedication to daily playing - you can’t stay in the top unless your on all the time no matter what team you have - just my observation as well experience).
However back to your original topic, yeah Guin stands alone in the center on defense for sure.
Yeah Dom would be a great recommendation for that! She is killer, but not so tuff - she needs SGG’s help! 
Honestly I have problems with Double HEL’s if Dark isn’t readily available on the board… Seems like the old HOTMs just can’t compete with the new ones very well. Now if Guin was sitting inbetween my HEL’s - geeez - watch out!
Or we can make all the 4* heroes trainable:
event’s heroes as Peters, Hansel, Gretel and Merlin counters everything pretty well but are locked in event’s summons.
Except you don’t get to the top 100 by defending, you get there by attacking. All we can see are the defences that the top attackers choose to set.
What do the top 100 attack teams look like? We have no way of telling.
Guinevere is not invincible, take three purple with you and bye bye guinevere !
And add two blues for Gravemaker 
Simply remove Guin extra defense against purple, she doesn’t need that to still be op…
I sometimes appear in the top 100 list. Once you get over 2700 cups you start getting raided constantly. It’s relatively easy to get in the top 100, but it’s way harder to stay there.