Hey!
So I am playing also cleric and for now I found these few tips to beat ark on hard level:
-farm Ashen Wastes/Sunken Citadel till you get ashen set + shadow leggins
-try to clear as many areas on main deck as possible, then while not spending batteries, recall, later you can exchange them for bigger batteries to stay longer in ark
-Farm reactor level until you get scalpel/sword,
conductive vest, goggled helmet, gloves (on the screen) and 2 star traveler rings,
-with these items as you can see your energy resist without any potions will be 137, which means wizards hit = 8, droid hit = 11 (with Battle Prayer III it's 4-5 and 8),
-skill sequence will be different in 2 cases:
* If there are no spiders, Summon Wolf, Battle Prayer, Arbenos Might and fight till the enemies are dead.
*If there are spiders, dont summon wolf until you tank them, then you can repeat above sequence.
-remember than when enemies get low, you can position yourself to the place where you are able to drag your wolf out of explosion,
-don't ever summon wolf on reactor level, radiation is right now deadly, oneshots
-If you don't need heal in actual fight, don't use it. Using heal takes time of ~2 of your hits, which significantly lowers time taken to kill enemy. So heal just in danger cases, in others, stay in a place after fight and heal yourself as much as you need,
-try to drag 1-2 enemies in each room to make fights easier, don't rush every place like a Rambo
Good luck man, let us know how it goes
Edit: and this set of skills works pretty well with me: