Unless you have something specific in mind, I'd make sure EF is in there by the time you reach level 14.
3 slot it with any endurance reduction TO/DO as its just too good to be left alone pre level 22.
Lingering Rad I tend to pickup after level 20 whenever I try new builds on Radio and thats with less blast powers that appeal with /Rad.
Acc Meta is best used as x3 rec red and x3 end mod.
I'd still stick an Acc on Explosive Arrow since its acc is lower than the others in the set just to be safe and for the moments when the toggles go down 
If you solo, EF is good ; you'll kill so much faster you'll save end in the long run.
If you team, from 6 to 20 Vigilance should help you to have RI + EF always on. At low levels, scrappers/tanks haven't good defenses yet and they will get hurt.
I always grab RI at 1, EF at 6 and LR at 12 on my rad defenders ; getting the "big three" so soon (and AM at lvl 2) is another thing that makes rad such a great and fun set, IMO.
I've already got to 6. I took Snap Shot, RI, Fistful of Arrows and Accelerate Metabolism.
Currently I'm lacking on the single target damage dealing, but I'm planning to get Enerving Field and then Flaming Arrow ( Superior damage at level 10 ) as my next 2 powers.
Fistful of arrows is pretty good, it's a 20-30 degree cone with a pretty good range. Using it on large spawns of RI'd blues and whites in the Hollows seems to work well.
I'm hoping that they will be enough attacks until I can pick up Explosive Arrow.
I did read that Archery has a long weapon draw animation, however it seems that even when using Rad powers I keep my bow in my left hand.
When do you suggest slotting Accelerate Metabolism, currently the plan is to have it slotted quite late?
Also what do you think of the slotting priorities of Radiant Aura, it seems a really poor heal.
@Unthing ... Mostly on Union.
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When do you suggest slotting Accelerate Metabolism, currently the plan is to have it slotted quite late?
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At least get the 3 recharges in there asap, and the 3 endmodif when you can (sooner rather than later though).
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Also what do you think of the slotting priorities of Radiant Aura, it seems a really poor heal.
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Likewise, I'd say get 3 Heals quickly and a couple of recharges whenever you can.
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
Radiant Aura I didn't pick up till 30 and slotted a bit later, focused on the debuffs (if they cant hit you or hurt you then you dont need to heal)
EF is a must IMO, think of it as a 25% damage and resistance increase to yourself if you want, then with the boost from AM.......... if you're looking offender, then you need EF
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Would agree with Okton on AM slotting, get the recharges in there as untill SO's the end recovery is very minimal.
Rad Aura I've typically taken at level 6 and for a long time had it at x3 heal and 1x rec red. Think I stuck the final slots around level 35.
Okay, I have my RI, EF and AB with 3 slots along with Fistful of Arrows, next level is Flaming arrow and then I begin the long haul to stamina at level 20 and can start adding additional slots.
I found that the heal really doesn't seem much use it also causes me to put my bow away unlike the other rad powers, however the debuffs and damage seem to work well both in teams and solo. Vigilance really helps the end cost of the 2 toggles, I'm looking forwards to getting to SOs now.
@Unthing ... Mostly on Union.
well doesn't matter if you don't have to slot for accuracy, you can't gain anything from 6 slotting with damage anymore, and archery does terrible damage anyway not to mention on a defender. Archery on defenders really is pathetic damage, like most defender damage although archery is more heavily resisted making it worse.
As for rad yes take enervating field asap along with rad infection and lingering rad..they are great powers.
Oh and accelerate metabolism asap to, slot it 3 endurance mod and 3 recharge.
Not having to slot for accuracy is good. It means I only have to use 3 slots rather than 4, which means more slots for other powers. However it could be argued that any rad build could do this given the def debuff on RI.
I'm quite liking this character, I suspect that is more to do with the primary than the secondary, but I am waiting for my next power, Flaming arrow which does more damage than a snipe according to Sherksilver's Hero builder and most of that is fire damage.
@Unthing ... Mostly on Union.
I thought I'd build a Rad/Archery defender to see what it is like. I'm planning to be a bit offendery, so slotting Rad infection ASAP and then getting AoEs and high damage attacks ( Fistful of Arrows and Flaming Arrow ).
The high Accuracy of Archery and the -Def of Radiation Infection should mean not having to slot Accuracy in any of my attacks.
Is it worth getting Enerving Radiation before SOs? Given that I would slot with end reds ASAP or should I hold out to Lingering Radiation instead.
For Accelerate Metabolism, do people slot it with recharge and end mods or just recharge, is it worth heavy slotting pre SOs?
@Unthing ... Mostly on Union.