Kassadin Tank + Attack Speed is a very underrated champion, this guide isn't trolling but it works really well.
In dominion he is able to tank (a lot) and deal a lot of damage thanks to his passive.

Mark - Atk Speed (obviusly)
Seal - Flat Armor (more tankish early)
Glyph - Magic Resist x lvl (wit's + odin's + passive + runes = their caster will do 0 damage and increase your atk speed)
Quint - MOAR ATK SPEED

Thornmail 1st item because it's pretty op in dominion and to counter all the physical attackers
Boots: Sorcerer's Shoes all on-hit items and W do magic damage and anyway endgame you will have 2.5 atk speed
Odyn's Veil (10% less damage taken from magic attacks) + Passive (15% less damage taken from magic attacks) + Magic Resistance = op :3
Now it's time do some damage, Ionic Spark (hp+atk speed) Guinsoo Rageblade and Wit's End.

R > W > Q > E

E just for the slow, Q just for the silence and W because it deals a lot of damage with his passive and his on-hit items.

21/9/0

Damage from offense tree and only the 'Good' masteries from defense

Ghost // Exhaust // Surge // Ignite

Any combination works well, usually I take Ghost/Surge or Ghost/Exhaust

PROS
-Can tank
-Can flash trough walls
-Deals AoE damage
-Owns a lot others casters, but it can beat without too many problems all the Physical champions of the game
-Underestimated, but he's a beast in 1v1 or 1v2
CONS
-Until the first Recurve Bow his only source of damage are Thornmail and Odin's, but W has a high base damage
-People will think that you are trolling

In teamfights jump on the most squishy one on the enemy team and try to get focus by everyone while your team attack them. You need to get focused from their casters, because they'll do very low magic damage on you and increase your Atk speed by a LOT.
Also, they won't expect you to be so tankish (99% kassadins build RoA+full AP) and with your passive they will just increase your damage.