Okay this achievement is not as bad as I thought originally. Assuming Sam B isnt your main character, the best time to go for this is during your Life 4 Right runs. As long as you skip all sidequests that are skippable, you should reach Act II around 12-13. By this point should have easily been able to upgrade to Lvl 3 tackle as well.
What you want to do is keep your machete you got from saving Nikolai during black hawk down, and upgrade it, you are throwing away the run anyway so money doesnt really matter. Fully upgraded, give or take some minor discrepancies, at that level you do about 200-300 damage on the ram.
Now we get to the actual business. Where do I do this achievement? Very first enemy you see in Act II in front of the church. Perfect place, the Ram is actually trapped here, if you run out of any of the gates he cannot follow. Most of the other zombies here wont pay you much attention unless you attack, but if you want you can clear them out. So basically what you want to do is, stand in front of him, and as he get ready to charge, dash to one side, and he should hit a wall and get stunned.
Key part, run up to him and slash him 3 times. You can risk 4 but you might kill him. At 3 slashes you whittle his health to about less than 1/4. And tackle level 3 does enough damage at this low level so its not a big deal. If you only get in two swings, set him up again, and put in one more swing.
Now when you are going for the actual tackle I feel the best place is the front gate, it is guarded by benches on two sides, with an opening just big enough for the ram to charge through. Stand in front of the gate, and as he charges jump over one of the benches. He is set up perfectly for you know, back facing you, only one path to go, so hold down the stick to run, and tackle is an auto skill that will hit the ram as you run through.
Now it will take more hits, unless you risked a fourth machete swing in which case I imagine you might be able to one shot him. But like I said, you tackle and run through, and should run out of the gate where he cant hit you. You can then run around, let him follow you out and repeat.
Plus as a bonus, if you die, you will probably spawn close enough, I did, that he wont heal.
The most important point is you want to tackle the ram at the lowest possible level, because tackle never really does more damage as you level up, but the rams life skyrockets. At this low level, 30 dmg a hit is pretty decent, as the ram has no more than a 1000 hp by my estimation.
Watch your health, and if you need a breather, any of the three gates are safe zones.
The machete from Nikolai, is probably the best weapon, that you are guranteed to find on an Act 1 run, you might get something better, and have to adjust accordingly. But if you get the machete, about 3 swings, just monitor his life bar, you want it down about to about 1/4 or less.
As a bonus tip, Sam B has a skill that allows him to regenerate based on time, ie every minute he gets x health. So if you run out of health packs, just wait behind the gate, and you should slowly heal, and just goad the ram, by being close, but watch out for his kicks, they have a long range.