I already have Madame Serris maxed out and have a Stag Knight at R5.  I'm not sure if I should take him to 60 or not (I have the tomes for his skills if I do).  Madame covers pretty much everything Stag Knight would do and is void, so no affinity issues.  The only difference is the three turn cool down on the ATK and DEF down vs four turns on Madame.  What do you think, is it worth ringing stag knight to R6?

Both Serris & Stag Knight are useful; Serris is much better in Arena; Stag Knight is much better in Dungeon, especially Spider 20, I use him in most Dungeon 20. 

Madam Serris AI can be very fickle at times.

- Madam Serris AI will do Def Down if you have someone moving ahead of her doing Block Debuff Buff to your team.

- Madam Serris AI will do Def Down if the enemy has more than 2 Buffs on them.

You need those very specific things to happen in order for Madam Serris AI to do her Def Down move.

Otherwise, Madam Serris AI will do the Block Debuff Buffs move.

Most good players use Madam Serris in there Arena Offense because you can Manual her.

Most good players use Madam Serris in there Arena Defense only when they have the right set up.

If you don't have the right set up, You shouldn't use Madam Serris anywhere which involves autoing the fight.

Arena Defense - Dungeons - Anywhere the AI computer is controlling her vs. you.

This is why players who own Madam Serris will often build up other AOE Def Down hero's.

They use the other AOE Def Down heroes for Auto related content.

Again it all depends on the other champions which you plan to have on the team.

As far as Stag Knight goes, He is very good hero.

This is how I have Rated Stag Knight: 

5 - Arena Offense

5 - Arena Defense

5 - Tag Arena

4 - Clan Boss

5 - Faction Wars

5 - Spider 

4 - Dragon

5 - Minotaur

4 - Campaign

5 - Fire Knight

5 - Force Keep

5 - Void Keep

5 - Spirit Keep

2 - Magic Keep

I give him a 2 in Magic Keep because Stag Knight is Spirit Affinity.

Stag Knight job is to do Def Down & Atk Down.

If you fight against heroes which are Magic Affinity, He will do weak hits.

The weak hits will cause his Def Down & Atk Down not to land.

So he isn't great against those types of heroes.

The same kind of logic applies with Dragon Dungeon.

Dragon Dungeon Stage 20 is Magic Affinity.

You really don't want to use him there.

However, The reason I give Stag Knight a 4 vs. a 2 is because Dragon Dungeon has other Stages in it.

All the Stages in Magic Keep have Magic Affinity heroes.

All the Stages in Dungeon Dungeon alternative to different Affinities.

You want to keep Stag Knight away from the Magic Heroes.

Any Magic Stage you don't really want him in there.

Any other Non-Magic Stage he is going to be amazing in.

So yeah!