I played it over again and got a few more hints:
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×If you reach the orb, you won't need to protect the priest anymore, because the roach will get stuck.
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×Forget about the fire, the best strategy is to use the splitters to distract the roaches.
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×To clean the horde of roaches that you have in the picture, stay close to a splitter killing them, and switch to the Archer to protect the Priest when needed.
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×If you're endangered, step on the blue PP to distract the roaches.
A few more tries and you'll do it. It's not as hard as it seems.
The main rule: Don't waste any movement!
That's it amb.