Rebecca deals extra damage on every weapon hit equal to 10% of her Block, so anything that grants Block is worth more on her than on any other hero. Plate Armor ringed with shields and helmets is her damage and her defence at once, Silver Rings add Block on hit, and Infernal Shield is the PvE pick: it trades half your Max HP at the start for a large Block, then turns a little more HP into Block every 5 seconds.
Read more about block builds
All five loadouts bring Mimic, and two pair it with Geezer, because the build is only as strong as its finished armor and both pets finish it sooner. Against healing enemies, add Ice Reaper and at least two Bloodletting Swords before round 4. Against a final boss that out-damages the Block, hold Seiren's cushion worth 200% of max HP for that one fight. In PvP, swap Infernal Shield for Celestial.