#Java 10, 77 bytes (limited to 2³²-1)

a->{int i=a.length-1,j=a[i];for(;i-->0;)j=(int)Math.pow(a[i],j);return j%10;}

Input is an int[], output is an int.

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#Java 10, 82 bytes (limited to 2⁶⁴-1)

a->{int i=a.length-1;var j=a[i];for(;i-->0;)j=(long)Math.pow(a[i],j);return j%10;}

Input is a long[], output is a long.

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#Java 10, 177 bytes (unlimited)

a->{int i=a.length-1;var j=a[i];for(;i-->0;){var r=j.ONE;for(var t=a[i];j.signum()>0;t=t.multiply(t),j=j.shiftRight(1))if(j.testBit(0))r=r.multiply(t);j=r;}return j.mod(j.TEN);}

Input is a java.math.BigInteger[], output is a java.math.BigInteger.

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java.math.BigInteger has a builtin pow method, but only accepts int as parameter. This last answer uses a golfed variation of this SO answer as implementation of a pow(BigInteger, BigInteger) method.

Java 10, 77 bytes (limited to 2³²-1)

a->{int i=a.length-1,j=a[i];for(;i-->0;)j=(int)Math.pow(a[i],j);return j%10;}

Input is an int[], output is an int.

Try it online.

Java 10, 82 bytes (limited to 2⁶⁴-1)

a->{int i=a.length-1;var j=a[i];for(;i-->0;)j=(long)Math.pow(a[i],j);return j%10;}

Input is a long[], output is a long.

Try it online.

Java 10, 177 bytes (unlimited)

a->{int i=a.length-1;var j=a[i];for(;i-->0;){var r=j.ONE;for(var t=a[i];j.signum()>0;t=t.multiply(t),j=j.shiftRight(1))if(j.testBit(0))r=r.multiply(t);j=r;}return j.mod(j.TEN);}

Input is a java.math.BigInteger[], output is a java.math.BigInteger.

Try it online.

java.math.BigInteger has a builtin pow method, but only accepts int as parameter. This last answer uses a golfed variation of this SO answer as implementation of a pow(BigInteger, BigInteger) method.

#Java 10, 77 bytes (limited to 2³²-1)

a->{int i=a.length-1,j=a[i];for(;i-->0;)j=(int)Math.pow(a[i],j);return j%10;}

Try it online.

#Java 10, 82 bytes (limited to 2⁶⁴-1)

a->{int i=a.length-1;var j=a[i];for(;i-->0;)j=(long)Math.pow(a[i],j);return j%10;}

Try it online.

#Java 10, 177 bytes (unlimited)

a->{int i=a.length-1;var j=a[i];for(;i-->0;){var r=j.ONE;for(var t=a[i];j.signum()>0;t=t.multiply(t),j=j.shiftRight(1))if(j.testBit(0))r=r.multiply(t);j=r;}return j.mod(j.TEN);}

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#Java 10, 77 bytes (limited to 2³²-1)

a->{int i=a.length-1,j=a[i];for(;i-->0;)j=(int)Math.pow(a[i],j);return j%10;}

Input is an int[], output is an int.

Try it online.

#Java 10, 82 bytes (limited to 2⁶⁴-1)

a->{int i=a.length-1;var j=a[i];for(;i-->0;)j=(long)Math.pow(a[i],j);return j%10;}

Input is a long[], output is a long.

Try it online.

#Java 10, 177 bytes (unlimited)

a->{int i=a.length-1;var j=a[i];for(;i-->0;){var r=j.ONE;for(var t=a[i];j.signum()>0;t=t.multiply(t),j=j.shiftRight(1))if(j.testBit(0))r=r.multiply(t);j=r;}return j.mod(j.TEN);}

Input is a java.math.BigInteger[], output is a java.math.BigInteger.

Try it online.

java.math.BigInteger has a builtin pow method , but only accepts int as parameter. This last answer uses a golfed variation of this SO answer as implementation of a pow(BigInteger, BigInteger) method.