= Small balance mods =

Yes, with focus fire, you can eliminate enemies without much trouble, but you need to focus on one unit at a time, because otherwise, they will all die. You need to focus on the second unit, and so on.

Well, it depends. A tank will survive under focus fire better than a Seraph with Drain. As for the level of danger, Drain doesn't do anything at all until the unit is attacked. So, there are two options: quickly focus fire on the Seraph, and then vampirism doesn't matter, or don't touch the Seraph and focus fire on it closer to the end, then vampirism has some chance of helping, but only at the end of the battle. At the beginning, it won't have any effect, and therefore it won't be able to provide the advantage that other, more dangerous abilities provide from the start of the battle.

In general, I should note that in single-player, Drain is a strong ability because:
1) The AI doesn't know how to counter it;
2) It allows you to minimize losses from the final attack, which is relevant if there are several opponents.