Like Wispy said above, teamwork: you're gonna need it (unless they're bad, but you said thst wasn't the case, so...).
I just had a few Ranked matches on Underpass, and it was so, so easy to pick off individual attackers coming from the center (between the .52 Gal Deco's power and throwing Seekers over the ramp, I scored quite a few splats), so unless they're distracted, it's best to use one of the other entrances.
I prefer the right-side drop because people tend to forget about it (at least, that's my experience), so I get out easily, and can either pincer around to our side of the center, go into the alley where the Splat Zone is (only in Splat Zones, but you can still ink it in Turf War), and you can go after snipers who went through their left-side corridor very easily (especially if they stand on the grate, then just shooting up gets the job done).
The left-side corridor leads to a pretty nice sniper nest (Squelchers and especially Chargers almost always go here in my experience, though one time I saw a Gold Dynamo Roller flinging ink from up there; that was a scary sight) and is a good place to drop into the center (and a ood vantage point to shoot up into the Splat Zone, though I don't think you can take control of it just by shooting from there), but be careful, you'll probably get attacked by people who dropped in from their right side (or from your central entrance to the center, or from inside your corridor) while you're sniping (you can probably hit them, but you should still be aware of the possibility).
If you think (or know, with Point Sensors or Echolocator or Haunt) they're distracted, exercise caution and go through. Look out for Ink Mines on the other side of the ramp (I haven't seen that happen yet, but I'd do it if I used something with Ink Mine, so watch out) and Seekers/Splat Bombs/Suction Bombs/Inknados from the Inkzooka coming over the ramp (it's a good bet that thy're expecting you if you see that happening, especially the Inkzooka), then splat anyone you can. Hopefully your team will assist you, but that's not guaranteed, and be prepared to retreat if you think you'll lose a firefight or get ganged up on (though it might be too late to run if that happens).
Hopefully this helps, it's won me quite a few matches (mostly Splat Zones, admittedly, not sure how well this advice extends into Turf War).