Firstly, I assume you are talking about regular Sceptile, and not the Mega? The Mega has slightly higher Defence, but not by much, and the same Special Defence, so it wouldn't really matter that much either way, except the Mega has to hold the Sceptilite (or whatever its called) for its item.
Secondly, I just wanted to point out that you actually can have a Sceptile with over 250 Defence and 250 Special Defence. It would need a nature that boosts defence (e.g. Bold), perfect IVs, and 252 EVs in both defensive stats, but it can be done. With this spread, a Spectile at level 100 would have 251 Defence and 269 Special Defence. It's not practical or viable, and is a lot worse than just about any tank or wall, but I just wanted to point out it could be done.
Now to answer the question...
The problem with making Sceptile take more hits is that Sceptile is built to be a sweeper Pokemon, and by definition they don't take hits well. It has high Speed and good Special Attack, but to compensate for that, it has bad Defence (its Sp. Defence is actually alright, though).
Having said that, it could still be done. If you wanted you could invest in Sceptile's HP and Special Defence, instead of its Speed, and put an Assault Vest on it (still give it 252 EVs in Special Attack). This makes Sceptile a decent Special tank. Its Defence is low enough, I wouldn't really bother trying to patch it up - just don't leave Sceptile in against physical attackers.
But there are other Pokemon that fill this role a lot better.
Sceptile also isn't helped by its typing - Grass has a lot of weaknesses, and so Grass types in general are not that effective as walls (there are some exceptions, of which Ferrothorn, Tangrowth, and Amoonguss are probably the most notable).
Conclusion...
So, after all that rambling - in short I wouldn't bother trying to make Sceptile take more hits. It's like asking Weavile or Alakazam to be bulkier. There are ways of doing it, but it isn't that viable, outside of the surprise factor. They are all meant to be sweepers: that is fast Pokemon who hit hard. The downside to a sweeper is their frailty: that is where tanks come in.
