Here's a few tips to consider:
~If you find any rows/columns that have 0 Voltorbs in them, flip all the cards over. It doesn't matter if they are 1s or not; they'll all be safe.
~Similarly, if you find any rows/columns that are ALL Voltorbs, use the Memo to denote that row/column as such. (Sometimes these can be more helpful; for example, if this runs perpendicular to a row with only 1 Voltorb in it, you can flip all the other cards over.)
~Next, look out for any rows where the point value plus the Voltorb count is equal to 5, e.g. 4/1, 3/2, and so on. You can ignore these rows completely; your goal is to flip over all 2s and 3s; flipping over 1s won't do you any good, especially if it's uncertain.
After that, this does involve a little guesswork, but you need to figure out the most likely places where the high numbers might be. Here are some ideas:
~If you see a row that says 4/2, you know there will be exactly one 2, two 1s, and two Voltorbs. (You can check the math yourself.) Now, if you managed to clear off squares enough so that you know that four squares in that row are either 1s or Voltorbs, you can flip over the last remaining card and it'll be a 2.
~If you see a row that says 9/2, you know there will be exactly three 3s and two Voltorbs. Rows like this are EXTREMELY helpful, as 3s are rarely seen, and some rows may not be able to accommodate them. For example, if it crosses a row that says 5/1, you know that a bomb will be there, since a 5/1 row cannot have a 3 in it. (And since you isolated the one and only Voltorb, you can flip over all other cards in the 5/1 row.)
~Rows like 6/2 are very tricky. You know there are 2 bombs, but the three numbers could be either 1-2-3 or 2-2-2. If you can help it, try to avoid rows like this at first and work on easier rows to figure out. If you flip over a 1 or a 3 in a row like this, it will help, as you know that there has to be a 3 or 1 there as well.
~A row that says 2/4 or even 3/4 is VERY dangerous. You know that there's a number there, but four of them are bombs. If you see a row like this, this will most likely be the last row you'll work on. If, however, you figure out where the number is (this is easier with 3/4 as 3s are rarer than 2s), such as when it intersects a row with no Voltorbs, it becomes a bit easier.
A lot of it does require guesswork, though, but a lot of it is technique. However, if you do need to make a guess, try to make an educated one. For example, if there are two rows that say 8/1 and they're perpendicular, try to hit the panel where they intersect; more likely than not it'll have a 2 or 3. (This method is not foolproof though; I've done this a few times and it has been a Voltorb, thus destroying my streak immediately.)
Sometimes you will reach a point in a level where you will have 4 squares left or so. You know that two of the squares are 2s, and that two of them are Voltorbs, and that the 2s are diagonal to each other (as will the Voltorbs). In that case, you have a 50% chance of clearing that level...and the only thing to do there is just guess. There's no way of knowing in that case.
Here are number combinations to consider; the first number will be the point value and the second number will be the Voltorb count:
4/2 = 2, 1, 1, 0, 0 (a 0 means Voltorb, and these numbers can be IN ANY ORDER)
4/3 = 2, 2, 0, 0, 0 OR 3, 1, 0, 0, 0
5/1 = 2, 1, 1, 1, 0
5/2 = 2, 2, 1, 0, 0 OR 3, 1, 1, 0, 0
5/3 = 3, 2, 0, 0, 0
6/1 = 2, 2, 1, 1, 0 OR 3, 1, 1, 1, 0
6/2 = 2, 2, 2, 0, 0 OR 3, 2, 1, 0, 0
6/3 = 3, 3, 0, 0, 0 (useful since you know only 3s can exist here)
7/1 = 3, 2, 1, 1, 0 OR 2, 2, 2, 1, 0
7/2 = 3, 3, 1, 0, 0 OR 3, 2, 2, 0, 0 (if you see this row, the best thing to do is figure out where the 3 can go)
8/1 = 3, 3, 1, 1, 0 OR 3, 2, 2, 1, 0 OR 2, 2, 2, 2, 0 (high chance of drawing a 2 or 3; is a good row to guess on)
8/2 = 3, 3, 2, 0, 0 (if you see this row, try to figure out where both 3s are)
9/1 = 3, 3, 2, 1, 0 OR 3, 2, 2, 2, 0
9/2 = 3, 3, 3, 0, 0
Any row that has at least 8 points in it is not only a good place to make a guess or two, but it's guaranteed to have at least one 3 in it. I stopped at 9 points, but I have seen rows as high as 12/1 (namely, four 3s and one Voltorb). Generally those rows are the safest, and they get safer as you get to higher numbers, but don't blindly guess; if you know it intersects a row like 3/2, then don't guess for any squares in the 3/2 row; as it's impossible for it to have higher than a 1.
The game goes as high as level 8, with 8 effectively being a secret level (7 is usually the highest reachable and I'm not sure of the conditions to unlock level 8), but they definitely don't get any easier. However I have noticed the chances of there being 1s on the field steadily decreases as the level increases.
