Your biggest problem won't be hp, def, or even dt. Your biggest problem will be swapping gear for precast/midcast. That is when your defense and hp will plummet.
Rdm does get access to all the stoneskin gear items which helps buffer the hp and with composure off the recast is extremely low. But, using all those pieces means lesser defense during cast. Even with cocoon, you are talking probably 1.5k def when using certain spells.
The HP isn't too hard, there are some pieces out there that can get rdm like 2.7k+ hp outside escha, but you are talking despair gear with hp augments, blistering sallet +1 and being #1 rank unity, odnawa earrings.
Sadly, you are probably better off stacking high enmity in certain debuffs and even utsusemi ni/ichi, otherwise you are going to kill your own survivability. (even then, you might be lowering survivability with rdm enmity options. some of the highest enmity pieces rdm can get are non-ilvl)