If you want to play an offtanky Warwick soltop, magic dmg onhit warwick is a pretty strong build.

For Runes i recommend Attackspeed marks and quints, because you will go for much onhit effects, so atkspeed is always good. For seals i take armor and glyphs i take mr / lvl, this will make you pretty tanky, if you face much early magic dmg, you may also pick flat mr glyphs. The Marks may also replaced with mpen marks, aswell as the quints, but you can also go tanky with those.

I go for tanky masteries, so you will get more sustain on top, and will make you more tanky for lategame, mastering both summoner spells, as you dont need the armorpenetration.

For items, i recommend to start with a dorans shield or a dorans blade, giving you more sustain, or boots of speed and some health potions. For boots you should either take berserker grieves, for mor atkspeed, so more hits for your effects, or magic penetration, because you will then deal more dmg with your onhiteffects. You should either start with wit's end, for the atkspeed boost and the first essential onhiteffect or madred's bloodrazors, which is your other essential onhititem. Then you should go more tanky, grabbing a guardians angel or / and a sunfire cape. Once you finish those, you may pick up a malady for another onhit and the magic resistance shred, a spirit visage or a frozen mallet

For summoner spells i take teleport and ghost, teleport for not missing farm and exp and porting to wards in tricky situations, recommended for solotop in general. As a second summonerspell i take ghost, so you can run away from your opponents or chase them even more effectively, becuase you max your e last.