The best way to learn a champion's weaknesses is to play that champion until someone beats you. That will show you how it is done.
This is not always an accessible solution - most recent champions cost a lot of IP. But in this case, Tristana is very cheap and there is a permanent promotion which will unlock her + a skin for free.
Writing speculatively, rocket jump has less base damage than explosive shot, but gains 5 more damage per rank than explosive shot does & deals damage up front, so the all-in style of Tristana play likely prioritizes ranks in jump. This means that anything which allows you to get out from under the jump and avoid its damage will save you from death. Rocket jump's animation is long enough to permit reacting with flash, but that has a much longer cooldown than rocket jump. Champions Ezreal, Lucian & Corki have their own move skills which act quickly enough to get you out. Caitlyn also has a move block but there's a windup animation, which may prevent you from getting out in time to avoid the impact damage.