Does Might go to minus? In that case, if I use a combat trick on a unit, how does it go?

Yes, Might can go to negative. Rule 143.2.b explicitly allows it — the game tracks the actual negative value for calculation purposes, but treats it as 0 whenever a spell, ability, or combat damage step references it.

So how does a combat trick work? Say your unit is at -3 Might and you play a +4 pump spell on it:

1. The game looks at the actual value (-3), not the "treated as" value (0).
2. It applies the +4: -3 + 4 = 1 Might.
3. Now when combat damage step comes, that 1 Might is what gets summed and assigned.

If instead you only gave it +2, the unit would be at -1 Might. For combat damage it'd contribute 0 (since negative is treated as 0), and if it also had any damage marked on it, even just 1 damage would kill it — because 0 Might means any non-zero damage is lethal (Rule 143.2.a).