An assassin's chance of success depend entirely upon his desire to escape after he takes his shot.
A suicide squad could probably kill any world leader, but a group requires planning, and that is a serious security risk for any conspiracy.
Yup. Armored truck bomb with enough explosives will most likely work.
Beyond that, though, I would say it depends on the skill and weapons of the squad.
Find someplace where you know he's giving a speech outdoors. The day before the speech your team takes over a bunch of apartments/condos overlooking the area, perhaps half a mile away. (You want multiple places but clustered.) They have heavy-caliber sniper rifles. They set up but behind lightweight drapes and a closed window. On a timer small explosives cut away the drapes and shatter the window, the snipers fire. You want the shots spread over something like a second so if there's any armored glass in the way the first rounds shatter it and the latter go through.
Yes, the guards very well might see the windows blow and know there's trouble but they have to observe it, recognize the threat, radio that recognition (it can be a single word but that still takes time) to the people near the target who then have to react to get him out of the line of fire. If the snipers are already aimed this will take more time than they have.
Second option--long before the speech they install some "machinery" on the roof of suitable buildings. So long as it looks like typical roof-mount stuff (say, air conditioners) only someone who knows what should be there (and how often is such a person going to be there??) is likely to notice it. They have a firing port built in. This takes more leadtime but won't tip off the guards.
Third option--similar to #2 but instead of sniper rifles they have something like the M-47 Dragon anti-tank missiles. (Not the modern self-guiding stuff.) The shooters can be behind armor plate that will make it unlikely a defender can hit them while the missile is in flight. This time you want a bit more spread so the missiles don't fratricide. Half the team aims for his feet so if the agents manage to dogpile him the missiles don't simply fly on past.
Fourth option, lots of leadtime here. Pick a city he's sure to fly into. Rent space adjoining every road out of the airport. (Looking over the local airport this would involve three locations to basically guarantee success. All three have nearby business locations.) Dig down and under the road. Place
large explosive charges under the road, seal up the tunnels long before the target is expected. When the target drives over one of your hidden bombs, push the button.
Sure they would use bomb-sniffing dogs but the dogs aren't going to detect the charge buried a few feet down below an impermeable surface and you allow plenty of time for any scent that got transferred in your building to dissipate. (You also make it as hard as possible to smell in the first place--the charge is freshly sealed elsewhere, then transported to it's final location as quickly as possible. Of course it's going to have to be transported in multiple pieces but that's just logistics.) I doubt 10-20 people can do this but most of them aren't combatants and aren't anywhere near the scene of the crime when the boom happens.
