What's your opinion on a Christian staying behind to assist the tribulation saints?
It won't be an option for any believer. It is God who determines who goes up in the Rapture and who is left behind. All true born-again believers will be taken in the Rapture before the Tribulation begins. The Tribulation is not going to be a time for which any Christian would want to remain behind. The vast majority of those who come to faith during the Tribulation will die and suffer under the Antichrist's reign.
Now is the time to be a witness to the world. Hopefully, the seeds that are planted now in the hearts of the unsaved will quickly germinate during the Tribulation in those who are left behind.
"For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever" (1 Thessalonians 4:16-17).