When I was using Meiling to tank Shikieiki's Last Judgment, I prepped in advance by pumping maybe 80 level's worth of level up bonuses into her HP, on top of all the HP skill points and HP items. In fact, I believe I started putting level bonuses into HP as soon as I decided I was going to challenge the Plus Disk with my team. I don't think you can reasonably take it if you only use skill points and items, at least not without extra level grinding.
As for Agastrobrauma, I had fun just tanking him with Iku in slot 1. Super-high MND, near invulnerability to PAR, and naturally good WND affinity? It's almost like she's tailor-made to tank that boss.
If you can fight Agastrobrauma, you should already be able to unlock Renko. Just use Renko's Debilitate to counter Strengthen Jutsu each time (or, hey, just keep applying it regularly). The great thing about Renko is that she doesn't scale with stats at all, so you don't even need to invest any skill points in her, and she'll still perform her job well enough. She just needs to be fast enough to switch in, cast Debilitate/Galaxy Stop, then switch out again. I just put her level bonuses into SPD and give her some random SPD-boosting items.