[Baalz]

Yes, indeed the primary idea on my build was "what's the cheapest way I can field something capable of taking out most PD?". Consequently (as I mention in that thread) everything they've got is necessary for this role - including regeneration if you're focusing on cost and not fully outfitting every one. This also includes their mistform, which means they are poorly suited to go up against anything with magic weapons in their PD. R'yleh is a very poor target for this as you noticed, as is a typically fire blessed Mictlan with PD over 20. Most other nations though you can take out an arbitrary amount of PD with just a frost brand and vine shield. WRT getting water gems initially you could certainly get unlucky, but sending a few of your water mages site searching early on should get you the small income you need.

When the game progresses and you start wanting more SCs than thug raiders you probably want to focus more on the Tuatha than the Sidhe Lords (after all, how many can you afford to equip with all that?). Depending on their randoms they can cast iron will/iron skin or quicken self/breath of winter (get more reinvig if you're quickening). Rainbow armor I found works very well at this stage, the protection is sufficient and reinvig/MR are both critical. Properly outfitted and with the right buffs for the specific job, Tuatha start hitting real SC levels.