Hello fellow KB player
1) about your randomness comment : this whole game is random, every time you start new game everything is randomly placed - units, items, enemy troops . . . You know where you`re going, and what is at the end, but road to there is your choice and depending on what game gave you, not what you want. So randomness is part of charm of this game in my opinion.
2) percentages %, and number ranges are also great part of planing every move in battles in game. crit% , resistance% , and numbers initiative, dmg, HP, speed . . . So you should know all about your units and enemy to plan good battle with low loss.
3) this comment i`m not saying you should forget no loss play, but you said you play normal so try to get to know what everything do in game, and try no loss/low loss at hard diff lvl it is much more interesting and challenging to finish then on normal. I say this because normal you have fast rage spirits and hero advancement and much gold and relatively weak enemy troops, so play it to learn about game and what every unit /spell/skill can do. Later on higher diff lvl`s spirits/hero gain exp lvlup slower and you must calculate moves better to finish game at all, and not just breeze through it.
4) you didint mention are you playing normal game, or some mod, so i`ll just recommend this mod to try it ( it have better AI settings for gizmo, and changed some rage spirit skills to work to higher lvl`s, among many other tihngs changed and improved in my opinion)
http://forum.1cpublishing.eu/showthread.php?t=29080
And finally about rage skills you asked about (for detailed info try to find file : game folder/data/data.kfs/slime.atom for Sleem skills / lina.atom for Lina skills):
Poison cloud : good at start , high dmg overall but totally unpredictable and unable to control. On high lvl it have great dmg and last long but all in all not worth it. As you mentioned stack of plants with magic shield and magic spring under it will generate huge amounts of mana for you, same goes if you use undead units, they will take low dmg from poison cloud but i dont think there is much use for it during whole game. On max lvl it makes 300-600 dmg, 13 times, for 16 rage, rest 3 turns.
Gizmo - unable to control same as cloud but have great option to resurrect units. It works on % like many other things in game, and if enemy unit in it`s radius higher leadership value than some of your units it will attack it, but if you cast it in background near your range troops and keep only 1 unit near enemy to block it while gizmo revive your lost units it will work great for you. Also in situations when you need to revive couple units for some amount it is better to have gizmo then cast 3-4 times resurrect spell for lots of mana. And gizmo is only way to heal black dragons in original game (not in mod i mentioned above). In end gizmo is a must for me, while is almost skip frozen orb and ice shards (Line is hardest spirit to lvlup because her skills make lowest amount of exp of all rage skills). On highest lvl gizmo make 1600-2500 dmg, heal 700 , 2 times, -1speed enemy/ +1 speed ally, act as dispel spell 3rd lvl, for 25 rage, rest 2.
So Gizmo is YES, Poison cloud is NO