I have similar issues, and I have been "analyzing" the escenarios along with the options robin has...
Ok, I have classified the mental state of the defensive players in 4 diferent ones ordered by conplexity (imo): the defensive player that relies on shielding, the one that runs away, the evasive and the walling style...
Ok so we have those mentalities and you need to study the first seconds of the match and find out which one your opponent is as soon as you can, and if at some point during the match they shift gears, you need to keep eye on this changes...
Once you define the type of defense you are facing is time for the pressure, always play safe against this kind of players and if you are on the lead don't over commit, even run away if necessary...
Vs shielding dude: 1. throw arcfire along with thunder if you get lucky the shield damage would be enough for thunder to hit, 2. Aproach by walking and do preventives d.tilts, 3. If you are close enough do a running away arcfire/pivot f.tilt and if your opponents keeps shield up or do a running shield mix it up with a reverse foxtrot grab (I don't know how you call this, trot dance or whatever), 4. Nosferatu while falling or while they are in platforms, shield grabbers hate this one specially.
Vs the running away: 1. Try to keep a distance of a thunder away from your opponent and pressure with thunders (duh), 2. Throw sh moving away f.airs/n.airs to the air (bronze ones are good enough), 3. Do preventives d.tilts along with arcfires from distance advancing by walking a little bit, 4. Empty hops into preventive jabs if close or any thunder variation(for some reason works)
Vs evasive dude (rolling/spotdodge): 1. throw preventive arcfires and use arcthunder to catch them, 2. Walk and throw d.smash if close to the ledge to catch them, 3. Use sh n.airs and reverse n.airs, 4. Arcthunder and thoron can be used too from a safe distance, 5. Running away f.tilts can be used too, mixing it up with pivot grabs
Vs the wall: 1. Match up knowledge is necessary specially endlag duration of the moves your oponents throws, 2. Try to keep yourself close vs other projectiles characters and at least 1/2 thunder away against bad/none projectiles characters, 3. Try to outrange them with f.air, 4. Run away when you are unsure of what to do next while getting hit.
Books/levin sword are your best weapons to punish, so catch them, if they hit a shield catch them... use the z-drop attack to trick opponents... throw the book up use a safe move to approach a little and if you fail recatch (the book will help you covering rolls and limit movement).
Remember that against certain type of mentality nosferatu may be useless or arcfire, so you can spend all the uses to get a book without worrying about your move being unavailable...
Also once you get a hit they'll be force to do a move, so capitalize on it as much as you can, and punish misplays even with little damage... don't be afraid to return to neutral if you are unsure of a move hitting... and work on your safe edguard (thunder->f.air not a true combo but very safe), force a.dodges and condition them by not punishing them (or doing a weak punish) until they are at kill percents or have lost their 2nd jump.
Finally never get impatient, this is what they want, if they are not forcing you to make a move... then why should you over commit???... play safe, and if they keep stalling and you are winning take your time...
Long post... but this is actually a very good question...I hope my "notes" help you tipping the scales... and maybe some others give away their tips too...