At stage 11 it is also a function of how hard you are hitting the enemy...normally you should be able to nuke your way down...but you also have to protect your team...Coldheart and Royal Guard's damage depend on the MAX HP of the enenmy..so they are more effective in higher levels. Perhaps you can subsitute one of them for a Miscreated Monter (throwing up shields) or a Psylar (Turn meter control)...if you have one, a Reviver like Scyl would also be awesome....

As a rule of thumb the accuracy should equal stage multipled by 10.  Therefore, it will be likely that in higher stages some debuffs or turn meter depletion attempts from Coldheart will be resisted by spider. Can it be that the turnmeter depletion from Coldheart's Heartseeker gets resisted on stage 11?

In my opinion it is crucial that the big spider get as few turn as possible otherwise it gets really tough. Therefore, I have two champions in my team (Coldheart and Tayrel) which can reduce/deplete the turnmeter of the enemy.  Without them I would be not able to beat Spider stage 11 to 14 (stage 14 is the highest stage, which I can beat).

Bigun can probably wave clear on his own up until around Spider 14, so that should let you advance at very least. He's not too useful at higher stages, but at the lower ones his AOE attacks should wipe the mini spiders.

are these champs all 60?

Yes, all the above champs are 6* level 60. I am working on getting better artifacts for them

Do we know the stats of the Spider at each stage, for example at stage 11, what is the speed, resistance, attack and def of the spider

do we have a page where we can find the enemy stats for all bosses at each level?

Spider strats:

1. AOE Nuke.  Works best for low levels like what your on, but your almost to the HP burn strat thats much better and will last you all the way up.  

a. use all AOE champs with enough support to keep alive.  

2. HP burn strat.  Works well at mid all the way to 20.  easiest strat imo.  

a. use a negative affinity spider tank.  Spider 12 is Green.  Use a Red, defenseive champ, to tank the spiderlings.  Your Doompriest can do this if you can build her with high def/spd/HP.  I wouldn't worry about resist on low levels unless you can really pull it off.  you probably cant.  BEK is also red so make her HP much higher than the spiderling tank to avoid her getting hit.  APO and BEK keep tank alive while the HP burn kills the spider for you.    

b. Mute Trundas damage.  She can not be an HP burner and a Nuker.  They are counter productive.  In HP burn strats you will try to do as little damage to the the spiderlings as possible.  This keeps them alive to BURN the boss.  If you have enough TM reduction on the boss, you can  HP burn right away, if not wait for the first boss turn then do it.  She will need more ACC and SPD to do this job.   

c. Apothecary- no need for accuracy or resistance.  SPD/HP/DEF and crit if you can, but not needed.  High def because APO can be your affinity tank on red levels if needed.    

d. Do not use Royal Guard until end game and your trying to pick the speed of clearing up.   Replace RG with someone like Armiger.  He will keep the boss TM down.  Felhound in a stun set would be an ok choice to CC the spiderlings if your affinity tank is taking too much damage.     

e.  Coldheart.  High HP, ACC, 70% crit, Spd and rest into crit damage.  DO NOT build attack.  ACC>damage.  Her first job is to deplete the TM.  This requirs acc and staying alive.  Second job is nuke.    

F. Armiger.  Easy to get.  Spd/Acc/crit/HP.  

3. Speed farm.  Highly gear dependant.