Well, I'd say that SCH is a very potent nuker in Abyssea. I took it from 75-80 there as well, and never encountered much resists, even nuking in full damage gear. If you have decent gear and capped elemental skill, you shouldn't have any problem with resists. SCH/RDM is definitely the way to go in most situations - if you're nuking, it will give you defensive buffs (Stoneskin, Blink, Phalanx) and MABII at SCH80/40RDM. If you're healing, it's now overshadowed by /WHM due to Haste at level 40, but /RDM will still allow you to be a competent healer and a great nuker. Not to mention Convert + Sublimation make it very difficult to run out of MP. In the meantime though, since you are working on RDM (you will not regret it, it's amazing for SCH!), if you're called on to be a nuker, i'd say /BLM will do fine in Abyssea - since mages normally are going for killshots, you shouldn't get destroyed too much from not having Stoneskin and Blink.
Easy to get gear I recommend for a nuking setup:
Teal Body, Head, and Legs
Vicious Mufflers (can be solo'd by a SCH, or more easily duoed with a BLM friend) - Errant hands are decent and the best AH hands I can think of, Goliard Cuffs are also good.
Goliard or Yigit Feet are still the best feet, if those are out of reach at the moment, River Gaiters are good for Mithra.
2x INT rings, a Moldavite and INT earring - add a decent cape and belt (there are multiple depending on your budget/resources), and you should be good to go on XP level targets. If you're having resist problems, Elemental Torque is good, and if your resources can allow it, Lemegeton Medallion can be great for > 50% MP. If your resists are no longer a problem, Uggalepih Pendant for < 50% MP is great.
Most SCHs and BLMs will have multiple nuking sets. One for max damage (stacking MAB and INT without concern for M.acc or Elemental Skill), and another for resistant mobs (stacking INT and M.acc or Elemental Skill, with MAB as an afterthought). The reason for this is on most XP-level mobs, resists will not be a problem at all. As a matter of fact, it was common for SCH's to nuke in Morgana's Cotehardie, which has a M.acc penalty, because even with it, we would cap M.acc. However, on very tough mobs like HNMs, you need to stack every bit of accuracy you can to land without resists. No matter how much MAB you add, you'll do more damage with a full nuke in M.acc gear than a resisted nuke in MAB gear. It may also be practical to set up a hybrid build for mobs a bit tougher than XP mobs, but not like HNMs. Also, for the record, a rule of thumb - not necessarily accurate completely, but good for estimation - is that 2 INT = 1 MAB and 1 M.acc on T4/T5 nukes. Also, 1 Elemental Skill will add 1 M.acc.
Elemental Skill merits never hurt either, and will open up versatility for your SCH. I personally was 8/8 Elemental at 75, so that might have helped explain why I did not encounter resists in Abyssea at first.
Helixes are useful on NMs that won't be slept. They are very MP efficient for their damage, and also since you will only get hate for the initial hit, they are also enmity efficient. Generally, it is best to straight up stack INT for Helixes, as they have a different formula than T4 or T5 nukes (they share the formula with T1 nukes).
Since you have Sea access, one of the best ways to improve your nuking damage for very little cost is to obtain the appropriate Obis. By casting the appropriate storm on yourself, you can guarantee a 10% increase to all of your nukes with the right obi on, which is an incredible boost, and will help you deal more damage for your MP spent. Note that you don't have to equip Obis with Helix spells, as they naturally have the effect of any weather automatically having full effect, so for those, you are best off with an INT belt.
HQ staves go a long way, if you don't already have them. That's an extra 5% accuracy and 5% damage dealt, they may be expensive, but there is no other way that money could be spent otherwise to increase your nukes' effectiveness by that much. I'd recommend at least keeping a HQ for your top nuke, and Ice as well, if it's within your means.
Speaking of Ice, if you are seriously having resist issues, use Blizzard nukes. Aquilo's (or Ice) Staff boosts Elemental Magic Skill and INT, so it'll definitively help your resist rate. Also because of the stats on Aquilo's and Stormsurged Hailstorm, Cryohelix is often the preferred Helix spell.
When nuking, take into account how much additional Refresh you have (COR or BRDs in the party), and determine if you need to utilize Parsimony in order to keep from having to heal MP, or if you can keep going indefinitely using Ebullience with your nukes. Ebullience nukes, especially with proper gear and Obis, can be very, very impressive, but if using Ebullience forces you to rest, when Parsimony would keep you going like the Energizer Bunny, Parsimony will normally be the stratagem of choice.
Sorry about the massive wall of text, this was basically word vomit of my experiences with SCH. I hope something from here helps!
Edited, Aug 29th 2010 6:15pm by apresmoi