Without help from other cards, this combo will only work once; when you discard the Basking Rootwalla to put a +1/+1 counter on Lotleth Troll, and cast it for its madness cost, you'll end up with a Basking Rootwalla on the battlefield, so you can't discard it again unless you find a way to return it to your hand.
With just those two cards there is no repeatable combo, you are able to use the Troll as a discard outlet to let you use madness abilities, but it doesn't return the card to hand, so it can't be repeated, it plays that card as if you cast it normally.
There is a way to make this combo work infinitely, and that is through the use of a card called Cloudstone Curio and a second Basking Rootwalla. The curio would let you play the first Rootwalla for it's madness cost then return it to hand when you madness out the second one, allowing each rootwalla to infinitely "bounce" the other until you decide to stop the cycle by not bouncing or not using the troll's ability.
When you activate any ability, you must pay its cost(s). In the case of Lotleth Troll, that cost is "discard a creature card". So you must discard Basking Rootwalla. Once you've discarded it, it's no longer in your hand, so you can't activate Lotleth Troll again. You'll need something more: multiple Basking Rootwalla in hand, some way to return Basking Rootwalla from your graveyard to your hand (if you don't cast it for its madness cost), or some way to return Basking Rootwalla from the battlefield to your hand (if you do cast it for its madness cost).