Steelix DT/C analysis
My both Steelix have Dragon Tail/Crunch. First I was disappointed, but they're not too bad after all. Steelix is only useful as a defender. It looks cool and has high defense. Super effective attackers are fire, water, fighting and ground type mons. Quick moves: IT has STAB but NVE against fire and water mons, so that cancels STAB out. DT and IT has the same damage, cooldown, but DT has higher energy gain (8 vs 6). Same with the charge moves, Heavy Slam is not very effective against two of the most common attackers (Flareon and Vaporeon, likely), same damage (70), but Crunch is a 3-bar move which means it hits earlier and possibly more frequently, also the damage window comes quicker. Cooldown is a bit longer for Crunch though (3.20s vs 2.10s) but it's not very a problem for defense. I do realise that in practice these differences between movesets are negligable, which means a battle will last longer for like 2 seconds tops.
Answers
So for DT/Crunch, one would need to bring a fighting mon to resist dark or a fighting moves Poliwrath who would resist crunch and get Super effective stab (or water moves).
Bad news though. Even with DT/C, an equal level Lanturn with water moves beats a Steelix with dodge only charge moves and realistic dodge settings per Pokebattler. So do all of his other usual common attackers like Vape (2 and a half of them at equal level) or Flareon and others too I'm sure.
Steelix is not a good defensive mon or a bad defensive mon. He is just like everything else not named Snorlax or Blissey in or around the top 20 in total stats. Except maybe a little worse due to his attackers being so common.
That is certainly true. But by one standard or another I stand by my statement. He is just like all the other defensive mons not named Snorlax or Blissey. This idea that certain mons are great defensive juggernauts only held out by CP is a lie. I think it is only really true for Lapras who has incredibly good typing. We should not tell it and have others spend valuable dust on such a lie either.