This is always my problem whenever I claim bonuses with wagering requirements. I usually bust all my rewards before I can even complete the wagering requirements.
I’m aware that slot games with highest RTP and Hit Ration should be the one I use but the problem is there’s tons of slot games out there while it’s very hard to sort them out based on this category.
What specific slot games is perfect for slowly gaining wagering with minimal lose rate?
It depends and it is not the highest RTP that keeps you playing for long,it depends on many factors but I would say the top two are the average hit rate of that slot,a medium volatility slot is sure to keep you playing more than a high volatility slot despite this last one having the possibility of making you richer in a much shorter amount of time,second it depends on the frequency that the bonus feature hits,now if you find such a slot with great hit rate frequency,medium volatility slot and that it gives the feature bonus quite frequently this is the one you may want to play to get to the requirements.
I always used to play Sword and Holy Grail,Diamonds of the Realm slots from Play n Go provider to be playing really long with a 100 dollars balance.Be wary though,these slots only behave well when playing with minimum bet,the moment you increase it,they can eat all your balance in an instant.
That's what I usually do before playing the game using real money. 
