I'm not very familiar with high rate servers but I'll help as much as I can with my knowledge of paladins. I only play 99/70 lvl/job rates so bare with me.
I know for sure that Super or High rates have instacast, at least most of them.
Str: I'd say a very high portion as it makes up for the damage done with Shield type attacks. Although, Shield attacks are permanently Neutral damage you should add as much Str as possible.
Agi: For a Shield Chain paladin, I don't think Agi is necessary for this type of build. The stats would be more important somewhere unless you're a hybrid Sacrifice build. Other then that, shield skills are dex based so agi doesn't effect.
Vit: Here I would probably throw a lot of stat points into considering that paladins are meant for tanking and to have the extra Vit defense helps as well. Although, you don't want too much defense because you want the Reflect skill to do it's job correctly, so you want to return most of the physical damage as possible.
Dex: For a 99/70 server, we have a hard time hitting instacast but i'm not completely sure on high rates. I can't explain much here unless I knew more about it but in 99/70 I went enough Dex for a quick cast rate. And as for HIT, i used a HIT weapon (ie: Phreeoni/Howard card) and had no problem hitting Flee whores. Switching is basic knowledge if you use BM(Battle mode) hotkeys.
Int: Again, as for a 99/70 we didn't need any Int due to potions and such but it's always good to have a good amount for one less finger to spam a hotkey rapidly. So after a decent amount of Str, High Vit, Quick castt, I'd throw the rest here unless again, you're a hybrid Sacrife/Shield build which is completely normal in high rates lol.
Now I have some questions for you.
Q1: Is Magnum strike the one when it targets a single person and has a chance of stunning (Bash) or is it the one where it damages a few cells around you, knocking them back (Magnum break)?
Q2: How much does do you need to hit Instacast on your server?
Q3: Do you use Shrink and Auto Berserk?
Now some helpful tips.
For armor-wise, I would have to go with the Meteor plate which gives a handful of defense as well as gaining resistance to Stun, Freeze, and Curse I believe? It's a rather more effective armor I suppose for a Paladin and also very simple to farm. As to Valk armor, hmm I'd say it's an acceptable armor due to it's capability of indestructibility with +1 All stats but meh I guess that works too but Meteor plate is #1 in my book for Paladins.
For weapon, I use a +10 Pike (Which is easily obtainable as well as refining since its level 1). Pike because you have the Spear Mastery which increases your spear type damage and yes, additional damage increases shield damage. So a +10 Triple bloody boned Pike[4] is the way to go, hands down.
Whelp, that's the best of my ability for you sir. You probably knew most of this information already but feel free to respond and we'll go from there.