My second thought was, change the controls! The original arcade game has, to the left of the movement joystick, a rotating dial with 8 positions to aim your gun, just like Nintendo's Sheriff, and like Ikari Warriors would a few years later. MAME emulates this as a second 8-way joystick, which is fine, except this is mapped as the left joystick for reasons I can't understand.
I was never good at games like this - namely Ikari Warriors and Commando. I don't know if these games are generally considered to be significantly harder than average, or if it's just that this style doesn't click well with me, but I always found myself struggling to gain ground against large groups of enemies with the exact same abilities as myself, and taking it slow just meant they'd spawn behind me. This was my experience with Front Line as well. Playing aggressively seemed to work better than advancing with caution, but it's still perilous. I did realize at some point that your gun has a longer range than the enemies', and success is really dependent on that. You have unlimited grenades as well, but they're slow and unreliable against foot soldiers, and tossing one over a barrier at a group is just as likely to kill them all as it is to explode harmlessly as they shuffle a few pixels outside its tiny blast radius, wasting precious time as more of them close in behind you.
To my surprise, the first time I made it past the initial area and entered the Tank Zone, I found things much, much easier, and made it all the way to the end of the stage on my first try. The gameplay gets speedier, but the enemy tanks just seem to be more predictable than soldiers. Getting into a tank protects you from a hit, but even on foot, I found dodging shots and countering with grenades to be easier than fighting foot soldiers. The large tank is basically impervious to machine gun fire - just get out when it starts to smoke, and get right back in. The final turret at the end can't move and only fires in eight directions like the other tanks, making it a cinch to bait it into firing a missed shot, and then knocking it out with a grenade as it cools down.
My best attempt reached level 3 and scored 22,000 points, but the recording crapped out early on level 2 and I didn't feel much incentive to try again. Point values are doubled on the second round and tripled on the third - I don't know if this gets capped later on. You get a maximum of 5 lives; three to start, and another two for reaching 10,000 and 15,000 points.
GAB rating: Average. Sorry if this post feels phoned in, but I just don't have strong feelings about this game, and can't think about much to say of it. I guess it plays okay, albeit with a weird difficulty curve. The graphics are kind of ugly and the sound is torturous, though that may just be emulation - a video of real hardware on Youtube suggests convincing gunshot and explosion sounds, at the very least.