Entity Relationship Diagram - Entity depending on attribute
Here's one idea:

CREATE TABLE Employees
( empno ... not null primary key
, skilllevel ... not null
);

CREATE TABLE Supervising
( supervisor_empno ... not null references Employees (empno)
, empno ... not null references Employees (empno)
, primary key (supervisor_empno, empno)
);


If you want to guarantee the skilllevel in supervising:

CREATE TABLE Employees
( empno ... not null primary key
, skilllevel ... not null
, unique (skilllevel, empno)
, check (skillevel in ('High', 'Low'))
);

CREATE TABLE Supervising
( supervisor_empno ... not null 
, supervisor_skillevel ... not null
,    foreign key (supervisor_skillevel, supervisor_empno)
     references Employees (skillevel, empno)    
, empno ... not null references Employees (empno)
, empno_skillevel ... not null
,    foreign key (skillevel, empno)
     references Employees (skillevel, empno)    
, check (supervisor_skillevel = 'High')
, check (skillevel = 'Low')
, primary key (supervisor_empno, empno)
);