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Looking at these units, it seems that 50 HP might be too little for them – it would be roughly the same as for liches. These trolls are so bulky that it looks like they should be able to tank and withstand attacks. Although their damage, attack, and defense are high for a Tier 5 unit.
Just my opinion – around 70 for the non-upgraded version and 80-90 for the upgraded/elite version would be ideal, with a slight reduction in attack, defense, and damage.
He's almost like a cyclops, with 200 HP.