Hugh will mostly accompany you with his starter, which is not only evolved once by now but also level 20.  Thankfully, he does heal your pokemon as you go, so you don’t have to worry too much about getting worn down.

The sewer contains a variety of pokemon that you might expect- mostly Zubat and Rattata, though you will find an occasional Grimer, at levels 14-18.  Grimer is an aggravatingly effective tank, combining its poison typing and strong defenses with a movepool practically designed to frustrate and aggravate your opponent.  Zubat is noteworthy for turning into Crobat eventually, one of the fastest pokemon there is and a quirky flying/poison type that has decent durability from its resistances and a wide range of both support and attack options, provided you’re willing to use its mediocre Special Attack in tandem with its quite satisfactory Attack stat.  Rattata, by comparison, isn’t anything special, but while Raticate isn’t going to win any awards for standing out, it is a good solid quick normal-type attacker, and by comparison to Patrat/Watchog is only missing a few small tricks in exchange for better stats and better attack options.

Head North from where the stairs come down, as the walkway behind Hugh is a dead end, and then hang a left past the murky water.   Take the first turn to the South you can despite the Plasma goons lurking around the corner- there are a few things to do down here.  If you head far enough South, you can find a Janitor who wants a battle.

Battle: Janitor Felix (640 Poke)
Opponent: Lillipup Lv 16, Trubbish Lv 16

Across the bridge past Felix you can find a small nook containing an X Special (basically a small sum of cash, effectively).  Meander around the North side of that ‘island’ and you can have a brief conversation with a tired office worker who points out that the rest of what’s down here is just trainers.  This is really a good thing for you, since you can face them with Hugh on hand to patch up your team from any damage they incur at no cost.  Continue to the West and you’ll encounter a room leading to a pair of bridges North.