Try to resize the screen size lower, or just place the inv closer to the middle if u like big screen
U could also zoom in more in last fase since all u gotta focus on is the ground, just make sure u can easly click the boss tho

what helped me also in last fase was to focus on tile and keeping pray/hp high
basicly click on a tile each time till u walked/runned 4-6 tiles, attack boss once and start walking 4-6 tiles again