While the Demon of Lies fight wasn't very hard (I just attacked him and ignored the summons), be was twice as tough as the next strongest opponent in the wastelands. I used 1 minor restoration potion fighting Moriendor, 2 with the Demon of Lies, and that was it for healing potions in the wastelands (I did use a couple more restoration potion to recover mana for Repair and Lockpick). I had LifeDrain on my sword and maxed Survivor's Instinct upon entering the wastelands (to use up some skill points) and that was more than enough healing for most of the rest of the game.
It was partly my fault the wastelands were easy, though, since I reloaded for major equipment to get the best stats.
When I re-loaded the game to check my stats without any equipment, I figured 'no guts, no glory', cast Heaven's Blessing for the one and only time in the game (another skill I used some spare skill points on), and re-did the fight with the Demon of Lies. It took 106 minor restoration potions and a couple minutes, but I killed him barehanded, with no armour.
BTW the publisher rushed the release of the game, so the wastelands ended up being straight hack and slash. I agree with you, though; despite a bit of a weak finish it is a very good game. Larian did the last act of Beyond Divinity first, so if they ran into similar time constraints (they didn't) it wouldn't hurt the end of the game.