Strongman competitions are between guys to see who can lift the most. This is from my knowledge, but you should check with the guy that invited you to get more info on what it is about. To train for this, just do sets that focus on strength gains not muscle gains which means low rep high weight sets, such as 5/5/3/3/1. When you deadlift, you use your legs to start the motion, so to make them stronger, do similar heavy squat sets. Figure out what your 1rm is, one rep maximum, so you can measure your progress as you train.