RE: Let's talk a bit about Exploding Rats, comment vote for good advice.

He's very comparable to exploding dwarf.

-Where would you put it out of the deck to make it more effective?
Would be very useful to use in a ruleset with Equaliser. I think hes a pretty niche card to use. Depending on the league you play he can be a really good utility card to remove divine shields (once he has the redemption ability).
In general it's a very fast card so it will most likely always go first. So even though it has only 1 health it can punch out some good early damage.

-How would you combine it to make it stronger?
I think if you play without the ruleset of equaliser, then possibly with fortify if they are not going to use magic. Since then he can absorb some extra damage that might be essential.
Also some speed up + slow down to the enemy, so he could possibly dodge attacks.

-Which summoner would you use it with?
Plado Emberstorm or Qid Yuff.

-Against which source do you think it will be more effective?
As I mentioned before if the enemy team has quite some divine shield and you have the redemption ability, you can basically get rid of them at once.
Other then that I would to try to use him a bit. But for now I was using exploding dwarf instead.
Since he seems better (because of the piercing).