A sentence with stamina?
You need a lot of stamina to play soccer.
There is no adjectival form of the noun stamina. You would have to recast your sentence. For example: 'My friend had enormous stamina when she was running the marathon.' 'I have noticed that David's stamina has decreased over the past few months.' If you are taking the word stamina to mean high stamina (as opposed to being just a neutral synonym for staying-power, with no indication of how much of it the person in question…
You DO need stamina. Swim more and stop and rest less. When I was on swim team we did a lot of things that increased our stamina and our speed. Stamina comes first though and you can only do that by swimming more and pushing yourself harder. If you don't have stamina, you will probably not do good when you want to compete with someone of if you are in a competition.