1-slimes make ice (just like the water does) invisible which mean he placed big slimes here to make the swamp

=howto=> with sethbling stacking entities filter for mcedit and some nbtedit (you can find some nbteditors for mcedit which help you doing this kind of stuff) or filters especially made to modify mobs you can place giant slimes riding a minecart

2- the damage are given through commands block with testfor or just specific detection radius

=howto=> you can place some command blocks running the command /effect @a[x,y,z,radius] 7(which is the instandmg effect) 1(seconds, doesnt need a lot to kill a player) 10(pretty sure players will die from this)
(/effect @a[x,y,z,radius] 7 1 10)

3 he simply lower the rails under the minecart on which the slimes are riding on with pistons to show us the path

=howto=> place a piston under the block where your minecart is with a not gate, when redstone's triggered, it'll lower the pistons

(if some stuff i said arent the actual stuff its because i didnt looked at his system yet but thats what im guessing and i know it would do the job fine like so ^^ have fun with it ! )