Method #1 Kill Nikolai by placing a Proximity C4 on the road when he arrives to the Casino

Method #2 Kill Caleb by shooting the glass railing and watch him fall to his death
(Shoot the glass railing in his balcony before Caleb arrives and leans on it)

Method #3 Kill Caleb by the driving the Stromberg out of the penthouse

Method #4 Kill Caleb and Nikolai at the same time by shooting the glass floor during their meeting and watch them fall into the lava pool below.
(Guards on the 5th floor will be alerted and they won't shoot you unless you go on the 5th floor also sometimes the targets have a chance of not landing into the lava pool but they will still die regardless) 

Method #5 Kill or snipe Nikolai while he sleeps in his Casino Suite

Method #6 Kill Caleb by shooting him through the skylight glass above him

Method #7 Kill Caleb by placing a C4 on the BBQ while he cooks his steak and detonate it

Method #8  Kill Nikolai by hijacking the taxi that he is in and drive it to an isolated and hidden spot and kill him there.
