A) The above few already answered it. Buffs. And when I say buffs I dont mean the minor ones like Arthur buff / Element dmg up / Andromeda Def up and the likes. I'm talking about powerful ones like Stackable Buffs, Self/Party Burst up, Taunt + Counter, Dmg cuts, Barriers, High rate/Guaranteed DATA, Guaranteed Crit, Party Vigor, Party Berserk, Party Dodge, Kaiser shields etc. Notable examples that you already have: Poseidon, Cu Chulainn, Eros, Svarog AW, Titania AW. Later iconic examples: Ashirat, Pluto, Freyr, SSArtemis, Chernobog, Vishnu, Berith, Aether, Uriel AW, Mike AW.
B, C & D) I probably should make it clear that the ONLY 100% I own is Hraesvelgr. I do NOT need my own 100% to be able to kill everything and anything in KamiPro using my other element teams. You also do NOT need Light Nike to beat Tower as Light. All you need is to have played long enough for a invested grid and a deep enough roster. By roster, I mean ALL rarity, not just SSR. A significant number of whales got destroyed because they've either neglected their non-meta roster or simply run in without knowing how to use their sub-team hoping to roflstomp the stages with raw stats. As for why my Water team failed, I did not have the right hero weapon (I had Arthur sword cuz I was slacking on advanced water builds), and running a team with Aphro + Saraswati + Raphy + Poseidon (had to use Cthulhu/Ashirat for earlier floors) without a max grid simply was not enough dps to stun Dysnomia Phlox from spamming rage + trigger nuke back to back which completely overwhelmed my defense. Again, no matter how many times you ask me I will answer the same: You need a strong grid. No amount of defense can save you if you can't get out of rage phase within 3 turns. Water failing aside, I cleared Phlox just fine with my Light team, having enough firepower while defending against Dysnomia's mechanic as well as her brutal firepower. Afterwards I even find my team too defensive and could have went for a slightly more offensive line up which would have actually reduced the risk of units dying (and formation falling apart).